With introduction of the Meta, Felix, the Red/Blue Zealots, and the Blues and Reds, there is bound to be some expectation of returning RVB Characters. So let's look in them.

Shelia
Shelia, Caboose's first crush during the early season of the Reds and Blues. Many believes that Shelia will be in the story to act as an obstacle for Arcee to be jealous of, but I have no plans in bringing her in. Because 1.) She has never been mentioned again except in season 13&14 and she is still stuck in Valhalla. 2.) She didn't partake in the multiple adventures of the Reds and Blues regarding Chorus and the aftermath of it. So she ain't appearing anytime soon. Sorry.

Junior
Blarggity Blarg-Tucker, or Junior, the biological alien son of Lavernius Tucker. Junior isn't going to make any appearances soon, why? Well, for one, he all grown up and is an ambassador for the Sangheili Embassy. Also he didn't partake in the fight against Blues and Reds so he's not aware of recent events.

Leonard L. Church
Okay, I've seen many comments expecting Church to be coming back, and many think I'm planning to bring him back as mini-con for Carolina or Caboose or something like that because the Meta and Felix are alive. Well... I'll tell you this, I have NO plans on bringing him back. Sorry to disappoint the Church Fans. As much as I love Church (he's my second favorite character) but he is dead permanently, he's in the afterlife.

Also at this point Church really deserves a huge rest and peace after all the stuff he's been through, and I don't think he would take kindly to being dragged into a conflict that lasted millions of years which will add a lot to his stress.

AI Fragments
Some comment including the ones in PMs believe I have plans to have the AIs like Delta, Theta, etc be brought back to life and be made as Mini-cons for the Autobots or decepticons to use. But again, they are all connected to Church but they were destroyed because of that. Of course Omega is still alive in this story since he is in Doc's head even though Omega is a part of him, but I would like canonized the idea that Omega didn't want to be destroyed so instead of being a part of Church's sacrifice he decides to reside inside Doc's mind. Permanently.

Tex
Tex, is an AI fragment of Church so she's gone as-well. There's no explanation needed for her disappearance.

How they got there?
I have reason why Meta and Felix are alive in TFP is because, in my opinion, their deaths give out an Air of Mystery and left us wondering if they did really die or were just MIA.

Yes, I am aware that the Red vs Blue Wiki has canonized that they are dead, and for the sake of the story on how they got to TFP in the first place, I'll explain how they got there:

Meta
The reason for his return is because when he was falling to his death in the icy cold ocean below him, in a desperate attempt he used his Time Distortion Unit to save himself like how Wyoming does it to restart time whenever he dies. But instead of rewinding time like it did with Wyoming it just malfunctioned due to the lack of AI operating it and especially Doc damaging it with his Plasma Pistol from earlier. This unfortunate event has transported him to the TFP universe, and yes he has been transported to Russia and been staying there for a long time a year before even Caboose arrives.

Felix
When he was falling to his death, Felix attempts to use the teleporting grenade to escape his fate but when he threw it, the grenade had malfunctioned and thus transporting him to the TFP Univserse. And yes, he has been in staying in America for at least a months before Caboose's arrival. Also yes, I'm aware when Locus picked up his energy sword, it did confirm Felix's death but I would like to head-canon that if the wielder of the energy sword has been separated from its user by going through a different dimension, then energy sword will looking for another owner should the former be missing through dimensional travel.

As for Blues and Reds, and the Red/Blue Zealots, it will be posted on a side-story soon that will come on sooner or later, depending if I have time. There are still some RvB Characters that hasn't been confirmed to make their official appearance but you have to wait for their reveal or appearances soon in later chapters.

That's all I have to say and I hope you stay tuned for the next chapter.