Okay so I had this idea for a long time and I'm not really sure if I should make it into a reality but here is my concept for a gym leader. Now for context I'm going to have to explain their background before I explain their team to give you an idea on their person and why they have this many Pokémon. And how it will fit in a game or story maybe I'm not sure about doing a story yet.

Name: Raven Crowley
Type specialty: Flying
Concept and background: Due to there being multiple gyms in Kanto as hinted in the anime as there are badges we never seen in the anime and Gary having ten badges. The actual reason why I wanted to make this oc first started with a fic I found on ao3(I was only looking for group chat stories and found this one)were ash ended up meeting and chatting with legendaries and it lead him to skipping pewter gym first taking on cerulean instead, lead him to the fairy gym(an obvious oc but still interesting). Another idea came to me while thinking and it gave me this idea. Also if you are going to argue about type overlay ( I'll explain that in the next part) I'll remind you that Cheren and Lenora exist as gym leaders at the same time, so it doesn't count as argument.
You know on how some cities sit between the boarder of states and can count for being part of that state, well the same can be said about cities that are split between countries. So what happens when you have a gym that's between the border well let's see. The city is a mostly modern one with in the center a small old town that still seems to be lived in and is treated as like a small suburb, and a town hall in the middle a town hall. As for the gym it is a ways across the own hall standing between where the suburb and city meets. And to the left of it is the pokecenter and in the city is a huge plaza market for everything you could ever need. As for the outskirts, there seems to be many construction sites scattered around for the imminent future buildings and roads. Raven's city sits on the very border between Kanto and Johto at the base of Mt Silver, often used by travelers as a rest stop and a way to cut around the mountain, with easier trails and roads to to and fro from each region. One specialty of this city is the gym, now like other gyms it is  a monotype specialty, however there are key differences. Due to how the town is place the gym leader has easy access to both regions flying types, making her team very diverse, but that isn't what truly makes her special. You see due to the border, it makes it hard to decide which regulations are enforced due to the minor differences on how each region is run, however, weather this was mayor's intentions or an innocent mistake the gym was made directly on what is considered the border as there is no actual way to tell. Also due to the fact that at the time there was only Lance as the champion for both regions and constant misunderstandings/lost in communication in orders, the gym now count to this day, as an Indigo league badge and a Johto badge. Now due to this exploit of having a badge count for both regions you can imagine the amount of people who swarm to this city. I mean getting this badge means that not only will it count for the league you are currently in, but if you ever wanted to challenge the other region, all you really need is seven badges instead of eight. As such the city is thriving with the amount of people coming to challenge the gym, to the point of constant expansion to buildings and the city itself.
The gym leader on the other hand, not so much; as she is constantly getting swarmed by challengers from both regions it takes a toll on her. Constantly needing to prepare for new challengers she started to breed her flying types, and constantly catches new ones as to not overpower and make it unfair to the challengers, with her having her older pokemon train the younger ones giving less time for her to bond with them, much less sleep. She is constantly not without caffeine weather it be coffee or energy drink, and constantly running herself ragged. Eventually in the game(if she was in the game or if I make the story for her) after you beat her she will passed out, it was "hinted" heavily by the trainers in the gym that it could happen at any moment and are more than willing to have the gym shutdown for her own safety. As each interaction from the trainers they constantly worry for her, and are mad at the mayor and city council for having pushing her to her limit, and are also blaming the trainers(yes that includes you in this case) for relentlessly coming and not giving her a chance to rest a little and take out their anger on you. That's when you find out a lot of the older residents who where born, and raised here felt it was partially their fault Raven carried on like this for the sake of the city. Originally Raven was only a air show entertainer using her pokemon for sky shows even a little bit of a magic her or there, but at the time the city was a small town that was about to be abandon and bought out for land for factory and mine works. Not wanting her hometown to be destroyed she came up to the idea to petition on being a gym leader, to help bring life into the town by having more people come, giving the town more money to prevent it from being tear down. It worked unfortunately it worked to well. While it died stop the destruction, it left the town with newer problems, they needed more room for people as the mass amount of people coming her meant that they would have to expand and react newer buildings, causing much of the surrounding area to be terraformed, and the displacement of many pokemon. New companies came in for profit, and the tax well that practically tripled thanks to how big it's gotten. The peaceful town was now a loud, crowded, overpopulated, and still expanding city. It hurt many people, including Raven for things to change this drastically, and now some people have to move away as they can't keep up with the ever growing tax and city. All Raven wanted was a way to preserve her town, and instead to her, she turned it into an urban jungle that is swallowing her town and herself. It's why she keeps on pushing herself to the brink of exhaustion; on one hand she feels guilty for unintentionally causing the people she grew up with to leave, and on the other hand has to keep up with the gym, as to not disappoint the challengers and the cities greedy council who keeps on expanding the city for their new found profit.
Now depending on if this is in the game(or I make the first story in years since I actually wrote something) you can bring this up to lance as the pokenav option in the games if I make this a story who might not even answer or before you ever meet lance or never meet him at all, then you can go to town hall, following the enraged gym trainers, who would complain about what happen. With how this would go down the members would either deny fault claiming it would be Raven's own fault for letting it go to this or that they symphysis to what happen to her. Eventually when it does get out that Raven passed out, let's just say it's does end well. Right outside the town hall you see an ever growing crowed of people mad. They are the original members of the city from the suburbs hearing to what happen to Raven from the angry trainers, the had enough. Enough of the treatment they have had, as well as the neglected health of the gym leader, as the council only appears to be interested in monetary gain, instead of their people. Eventually as the crowd keeps getting louder and louder more people keep on coming gathering around to see what is happening. Before long as the officers come to break up the crowd, two loud voices sounded: one more ragged than the other, as the other seemed more younger, but still mature came up from behind the chaos.
When all eyes turn to see who tried to stop them, to their shock it is one of Kanto's own elite four, Agatha, and Johto's Karen. News of the exploit started showing up when the league officials notice the same badge appearing in both leagues. As they dig deeper they found out about the gym and how under misunderstanding got counted for both. Not knowing what to do, since while technically not their fault as no one corrected the mistake, much less realize it was even happening, both leagues decided to send a representative to the city and evaluate it again. To find out that the gym leader is unwell, and that thanks to the complaining people in front of the town, was made this way do to how greedy the city officials wanted to keep turning the city into a metropolis. They decided it would be best to shut down the gym, as it was obvious from the root of sorts that the officials were skirting around the rules just for money, however they will not punish it's leader. It wasn't her fault that things got to this point, and a part of her intrigue the both of them. Finding out from the complaining people, and more specifically the gyms trainers, that she was never able to truly use her actual team, as she her own kept on getting to strong for the gym challengers changing it constantly to keep up demand. An adaptable type specialist that not only pokemon, but can easily raise new ones into strong pokemon was intriguing. As precedented with Koga it is possible for a gym leader to be promoted to be an elite four member.
(I am mostly leaning to have her be a Kanto elite four, so that Lance or Bruno for Kanto, mostly leaning towards Bruno as I'm kinda sick of him just being the elite four for both of regions and who knows since Lance got demoted to elite four member in Kanto, but is counted as the champion in both games at first before Blue comes and demote him to elite for in the games, seemingly kicking out the original elite four member, also since I'm modeling this after Koga I'm also putting the no overlapping type specialty between elite four and gym leaders, as Koga is a Johto elite four, but his daughter is a Kanto gym leader with the same type, so thanks to Falkner, I have to make her a Kanto elite four, but then again Juan and Wallace from emerald proves it otherwise so depending on which game it is still almost guaranteed to replace Bruno as I hate redundancy of Bruno for both elite four)
As per the demands of both elite four the gym was closed, and Raven being free, now as to not push herself to far, on a much long needed vacation to not only regain her health, but to properly train, and build trust in her pokemon. However, the city itself is still thriving, while not having it's gym did slow down the flow of people coming to the city, the new roads and market, maker it easy for goodies to be transfer to and fro each region. Now besides a huge mall market in the city areas, there is also a bazaar, where you can: trade pokemon and items, buy unique items, participate in games for prizes, and watch contest, as the gym is now a contest hall.
When you finally see Raven again, she is looking well, happier and willing for your challenge, but this time as an elite four member.
Gym: The gym itself looks similar to an indoor stadium on the outside, but in the inside you are greeted with a room with three closed curtain that lead to different rooms but with an image of different pokemon. Before three different flying types come out of nowhere all at once with there, afterwards you notice a sign that wasn't there before, and written on it says,"Remember the pattern, for from here things are going to be risky." That's right the gym puzzle is a memory game, and with every set you pass, the next set will come faster, with one more added to the set as well. Now obviously the rooms were it's the wrong answer is where the gym trainers are the actual difficult part is the pattern. As, depending on which direction the birds are coming from depends on which way you go after exiting the room you where in as it would then branch into different hall ways, the right one will lead you to the next set of doors, but the wrong one will force you to go through a bunch of trainers, by making you go to the very end, only to turn back and find out that the hall you just walked through is now filled with trainers.(Look I wanted something similar to Blain and Fantina as it would fit how fun of a job she was as a performer, but thought it was to simple so I mixed in the element from the Sootopolis gym, where if you fail the puzzle you are forced to fight it's trainers)
As for trainers there are obviously bird keepers, however they aren't the only type of trainer here. There are many breeders here as a reference to the pokemon the gym leader breeds herself and are obviously on stand by in case something bad happens, as well as clerks from the city, who are here to promote the company they work for and gain interest from the people who come and go into the gym(albeit begrudgingly saying that they need the money to help deal with the ongoing tax raise, but still feel slightly guilty for the gym leaders health), and sky trainers who had preformed with Raven in the past and decided to help lessen the burden
Team: (as I said before she constantly changes to adapt to newer challengers so considering that you migh meet her after facing your first league and going onto the other region I'm making a standard gym team under the eighth gym leaders rules first then showing her team for the elite four)
Gym team:
Honchkrow

Noctowl

Skarmory

Crobat

Pidgeot

Elite four:
Honchkrow

Skarmory

Crobat

Pidgeot

Togekiss

Gyarados

(I'll decide their moves and natures later as I suck at remembering natures, I'm not a hundred percent sure weather or not I keep both pidgeot and togekiss as it is kinda redundant of having to normal flying types so let me know if I should keep the team like this or not)