Looking back on the golden days of the Gravity Falls fandom, I remember there being a lot of different theories ranging from being a bit ridiculous to actually being spot-on. Here's a list of some of the most memorable ones, along with the "proof" for it and my opinions on them (past and present, depending if my original stance on it has changed).

The Theory: Gideon is a vampire

The "Evidence": This theory was based mostly off of Gideon's appearance: his white hair, pale skin, and cape (yes, the cape, because clearly a cape is the sign of a vampire). After the premiere of "Blendin's Game", one believer of this theory even went as far to say that the graffiti painted on the sign of Gideon and Bud was foreshadowing of him being a supernatural creature, therefore supporting the Vampire!Gideon argument.

Emerald's Opinion: We're seen Gideon under the sun and in front of mirrors before. Yeah, my son is not a vampire. Leave him alone, he's trying to be a good kid.

The Theory: Robbie is a zombie

The "Evidence": "Known for their pale skins and bad attitudes, these creatures are often mistaken for teenagers. Beware Gravity Fall's nefarious zombies!"

Emerald's Opinion: While the similarities between Ford's description of zombies and Robbie are admittedly similar, I've always thought that Robbie was a bit too articulate to be a zombie.

The Theory: Bill is future Dipper

The Evidence: This theory surfaced sometime during the hiatus during Season One and Season Two. There were some dialogue and staging that the fans who believed this theory thought could be proof of this, like both Grunkle Stan and Bill doing the "eenie meenie miney you" thing to Dipper. Someone also thought that Bill/Bipper treated Mabel a little worse than Dipper in "Sock Opera", which they thought could mean that he had a personal vendetta against her. I believe the most popular backstory for this was that Dipper and Mabel would get into a horrible fight in the future, which ended in Dipper hating Mabel's guts, and when he got tangled up in some supernatural mess afterwards, he was transformed into Bill. It was believed that Bill-Dipper's motivations was to change the past.

Emerald's Opinion: No. NO. NO. FOR MULTIPLE REASONS.

The Theory: Mabel has a secret twin/clone named Lebam

The Evidence: This one was all started when a fan noticed that one of the bottle letters than Mermando sent Mabel was addressed to "Lebam" while the others were addressed to Mabel. Things got a little out of hand from there.

Emerald's Opinion: Remember when I said that some of these old theories were a bit ridiculous? Yeah, I was thinking about this one when I said that. Besides, Alex Hirsch himself confirmed that the whole "Lebam" thing was an animation error.

The Theory: Bill was still in Dipper's body

The Evidence: It was thought that Dipper being frozen in "Northwest Mansion Mystery" in the same pose that the Shapeshifter did in "Into the Bunker" meant that something was up with Dipper afterwards. After "Not What He Seems", someone used a screenshot of Dipper after the portal flashed that blue light and Dipper's body sort of got stretched out, along with his eyes, making them somewhat resemble the Bipper eyes, which was believed to mean that Bill was in possession of Dipper's body still.

Emerald's Opinion: I thought that it was established in "Sock Opera" that Bill cannot act like other people. Even though he managed to fool the characters as Bipper, it was always obvious to the audience that that was not Dipper. Even if you edited the eyes and replaced Bill's voice with Dipper's, Bill's human body language was very different from Dipper's.

The Theory: The Author is Grunkle Stan's secret twin

The Evidence: This one started to gain popularity after one Tumblr user made a long post with all the Season One evidence suggesting that Grunkle Stan had a twin: his car's license plate saying "STNLYMBL", the Stan-lookalike in "The Time Traveler's Pig", the secret room having a calendar from 1982, Grunkle Stan acting strangely sad over the glasses he found in that same room, etc.

Emerald's Opinion: ... Okay, I guess I have to admit this. I did not believe the twin theory when it first surfaced. I thought it was a bit cliché, and I brushed off all the evidence. Besides, there was also the "leak" that occurred that had me convinced that the Author was someone else (more on that in a minute). Of course, I was proven wrong.

(A quick story time: I watched "Not What He Seems" for the first time during Spring Break with my youngest cousin, who usually hates movies and TV shows, but he loved me enough to want to watch it with me because I loved it so much. The two of us locked ourselves in one of the bedrooms of the condo we were staying at with our family and laid at the foot of the bed to watch. At the end of the episode, when Ford was stepping out of the portal, I wiped away my tears- because I was totally crying during that entire scene- and sat up. "Wait a minute", I whispered as he picked up the Journal. "WAIT A MINUTE", I said a little louder when Grunkle Stan began the most iconic line of the episode. "WAIT A MINUTE", I screamed when he finished the line and Ford revealed his face, and then I fell off the bed. As I laid on the ground, shrieking and mumbling other words that definitely was NOT English, my cousin sat calmly on the bed, looking thoughtful. "Well," he finally said. "I think I may watch this show now.")

The Theory: McGucket is the Author

The Evidence: Ah yes, let your good friend Emerald take you on a journey of the most epic trolling.

First, this photo was "leaked" after the "The Author is Grunkle Stan's twin" became really popular:

After that, Alex Hirsch tweeted this:

He deleted it minutes later, leaving the fandom to believe that the photo was legit and that McGucket was the Author. Of course, this was proven to be false after "Not What He Seems". To mess with us even further though, Mr. Hirsch posted this:

Emerald's Opinion: I was fooled. I got bamboozled. I let Mr. Hirsch trick me like he did the others. Way to go, Emerald.

The Theory: Dark Dipper

The Evidence: It was believed that Dipper would go "dark" after the events of "Not What He Seems", feeling bad that Mabel would trust Grunkle Stan over her own twin brother. The glass of the photo of Dipper and Mabel breaking during that episode was thought to foreshadow this. The fandom seemed to LOVE this theory; there was so much Dark Dipper fanart during the Great Not What He Seems Hiatus™.

Emerald's Opinion: I HATED THIS THEORY FROM THE MOMENT I FIRST HEARD OF IT. GET THAT ANGST AWAY FROM THE CHILDREN, THEY DON'T NEED THAT MESS IN THEIR LIVES. Besides, I thought that it wouldn't make sense for Dipper to hate Mabel for not pressing the button when it brought him the Author of the Journals right before his very eyes. Luckily, I was right about this theory. THANK GOODNESS.

The Theory: Tad Strange is another demon

The Evidence: ... There really wasn't any?? This whole mess started when Cecil Baldwin added the name to his list of credits on his resume. During the Bill Cipher AMA, Bill answered a question about Tad by saying "Tad's a real square", which the fandom ended up taking a bit too literally. The fans created their own square demon Tad Strange, and even when Mr. Hirsch constantly shoot down the theory, some fans still believe that they are right about demon!Tad.

Emerald's Opinion: This was one of the funniest things that happened in the fandom. I vividly remember watching this whole mess escalate as I laughed at the fandom. Ah, those were the good old days. But no, I don't think Tad Strange is a demon. (Although in Journal Three, one of Bill's cryptograms revealed that some of the aliens from the spaceship crash survived and are living among the citizens of Gravity Falls as humans so...)

The Theory: The Author was the Invisible Wizard

The Evidence: This theory debuted during the Great Not What He Seems Hiatus™, before we knew Ford's name and where the portal led. Of course, this is also before Journal 3 was published with proof that the Invisible Wizard was real and not just a one-time joke. Once again, there really wasn't any evidence for this? People just took Mabel's line about an Invisible Wizard and went crazy with it. It was thought that going through the portal would still put you in the same dimension, but just in its dreamscape.

Emerald's Opinion: I thought that this was a pretty fun theory even though I didn't believe it. There was also some hilarious fanart of Ford reacting to stuff happening in the Mystery Shack over the course of the show that was always a joy to see.

The Theory: Stan's death will be by fire

The Evidence: All the other Grunkle Stans did not have a very good track record. Wax Stan got his head chopped off, Puppet Stan got set on fire, Hot Air Balloon Stan also caught fire, Grunkle Stan faked his death by setting his car on fire, etc. This was assumed to be foreshadowing for his own fate: death by fire. Besides, Mr. Hirsch also tweeted that at least one character was going to die in Season Two, so after that, everyone freaked out.

Emerald's Opinion: I did NOT want this to be true. Nope. Nope. It ended up being mostly true though, only without the death. *sigh of relief*

The Theory: Mystery Trio!

The Evidence: During the Great Not What He Seems Hiatus™, it was assumed that Stan and Ford had a great sibling relationship before Ford got stuck on the other side of the portal and went around Gravity Falls, saving mysteries together along with a young Fiddleford McGucket. The three of them were referred to as the Mystery Trio. Their adventures and misadventures were frequently speculated on in many different fanfictions and fanart.

Emerald's Opinion: Ah, this was one of the purer of the theories born out of the Great Not What He Seems Hiatus™. I wanted this to be true, but you know. It wasn't. At least we still have the memories of those days...

The Theory: Grunkle Stan is Grandpa Stan (also known as: #grunkle4grandpa)

The Evidence: At the time this theory was created, it was in between "Not What He Seems" and "A Tale of Two Stans", and the fandom only knew about Grunkle Stan and the Author. The Author being Dipper and Mabel's grandfather seemed to the obvious choice, but then a fan said, "wait a minute, if Grunkle Stan ends up being Stanley and the Author ends up being Stanford, would Grunkle Stan be Grandpa Stan?" The fandom freaked out over the potential fluff and angst this possibility could bring, and embraced it full heartedly.

Emerald's Opinion: This is my favorite non-canon theory because oh my stars, can you imagine how cute that could have been? I wanted #grunkle4grandpa to end up being true so badly, but even though there ended up being another brother ("WHO IN THE WORLD IS SHERMIE????", I yelled aggressively at the TV while watching "A Tale of Two Stans" for the first time), I'm glad this theory at least brought another bright spot to the fandom in the midst of all the post-"Not What He Seems" angst circulating around and the Dark Dipper stuff infesting every other Gravity Falls fanart.

Wow, I got hit with the nostalgia train while writing that. Whoa.