Natasha is gone for good. Her movie is definitely going to be a prequel of somekind, possibly explaining her origins in Russia.
Bart is also never coming back. He has a family, a family he lost and got back and he ain't going to waste that second chance. I don't see him ever doing superhero stuff again, especially with all that talk about how he became a monster.
I can see Rogers getting a cameo here and there. A really cool angle would actually be to get him to become the new director of SHIELD. That way we'd always know he's still there, working behind the scenes, but he wouldn't have a substantial role any more. The dude is literally 106 years old by the end of Endgame, though, so I can just as easily see him passing away from old age before the next Captain America movie.
Thor isn't going anywhere. Thanks to Ragnarok they finally got his character right and the last two Avengers movies really added a lot of depth. I don't know if he'd stick around with the Guardians of the Galaxy for more than one movie, but now that he's free of the throne he can just about go anywhere and do anything.
As for Banner, notice how he wasn't human once the whole movie? I don't think we'll ever get a Hulk solo movie but I think this whole Professor Hulk angle is Marvel's way of keeping Hulk around for as long as possible regardless of how old Mark Ruffalo gets. As long as he continues to be available for mocap and voice overs, he can continue playing Hulk for another ten years easy.
I do think Chris Hemsworth and Mark Ruffalo are the only folks from the first five years that are sticking around, though. I think everyone else is just done now.