It's hard to say for me who I'd want to be in it having no clue in what direction they'll go with it. What's the age rating? How integrated will it be into the MCU? Just how self-referential will they be about the transition from the FOXverse to the MCU? Will it be addressed through a quick "wink wink nudge nudge" joke or will we actually see him travel the multiverse as part of the plot or something? Will they have introduced mutants in the MCU before, will they start in this movie or will they wait until after this movie?
I wouldn't mind if they chose to bring over some supporting characters from the previous movies, if only to not completely start from scratch and have at least someone Deadpool knows. He just got Vanessa back, people really seem to like Negasonic Teenage Warhead, they just introduced Cable and Domino. I think it'd be a shame if absolutely none of that would matter and Deadpool ends up in a world without any friends having just lost everyone and we'd have to spend a chunk of the movie coming to terms with that and introducing a fully new main cast besides DP himself and try to make DP get to know them and form some kind of relationship from their first meeting. They don't have to bring everyone over but I think they need to take at least someone along.
I think people will want to see Deadpool interact with at least one beloved MCU character who can be the straight man to his antics or be confused by him. I could see a character like Nick Fury, Rhodey, Bruce Banner, Bucky Barnes, Clint Barton, Sam Wilson or Doctor Strange fill that role. Personally I think I'd give it to Sam or maybe Rhodey because I think that Anthony Mackie and Don Cheadle are good at being funny as the straight man and both characters could use another appearance to highlight the roles they'll fill in the MCU going forward (Fury and Strange in particular are already showing up in other peoples' movies a lot so let's go with someone else). Though I could also see them go with Ruffalo as Banner since the two recently worked on The Adam Project together.
There's also some more inherently funny characters already in the MCU that might lend themselves well to a comedic tone like Howard the Duck or even someone like She-Hulk. They will probably introduce Mr. Immortal soon and Deadpool has some history hanging out with him and the GLA in the comics (plus they could do a gag where they try to kill each other but can't keep each other dead). I think Korg might be particularly likely since Ryan and Taika have worked together recently, seem to be friendly with each other and even already did a Free Guy ad as their respective Marvel characters. If they actually continue on from the Deadpool 2 timetravel ending and have him physically transition to the MCU in some way I could also potentially see a TVA sequence with Deadpool and Mobius, depending on when it releases relative to Loki season 2.
As for introducing fully new characters there's dozens of them that could make for a good story or a funny gag, many of which have been mentioned in the thread already. I'll trust the team working on the movie to pick the best ones.
