I think it'll depend on how well Captain Marvel is received by fans. If well, then it'll be her, if poorly, then it'll likely fall to Black Panther or Spider-Man. I'd like to think I'm wrong, but I don't see them doing a whole lot more with Thor (at least as far as the Avengers go); from this point forward he's seemingly going to be underpowered compared to Carol.
Doctor Strange lacks presence. I don't know why, but the character seems somewhat reclusive (like an introvert maybe?); I think he'll play his part both within his own films, and within the occasional future Avenger films (depending on the threat) but I don't see him at the forefront. I don't think they'd be able to suddenly introduce someone new to take over the mantle either, not at this point. It's going to have to be a character that's established by the time Endgame ends, and I don't think a character introduced in Endgame is going to be able to pull that off.