Well after Thanos, the third Avenger movie could feature the Masters of Evil for enemies. You'd have Red Skull and Blonsky team up with what ever other bad guys show up in the movies.
Here is how I would do it: (Fan fiction mode... on)
Starting with the upcoming solo films and concluding with the 2nd The Avengers film, the team's stature is slowly picked away and people lose confidence with the individuals. During these films the Infinity Gems are slowly introduced (e.g. only "Mind" shows up in Iron Man 3), building up to Thanos.
Hank Pym is introduced and along with him Ultron v0.5 in the 2nd film. Make him extremely jealous and paranoid, but just drop the wife beating part. By the end of the film Iron Man loses access to his suits. Captain America loses his shield.
S.H.I.E.L.D. movie which features a new protagonist who seems likeable enough. During the film Fury is displaced (maybe framed so he becomes an outlaw) and Maria Hill takes over. With Fury gone, SHIELD is unable to prevent the disbanding of the Avengers. At the very end of the movie the protagonist we've grown to like and find relatable, reveals himself to the audience to be an agent of Hydra under the name of "Zemo".
Rather than go into "The Avengers 3", the next Avengers film would be something along the lines of "Dark Avengers" (an adaptation of that series, more or less). Zemo goes full Iron Patriot mode and remakes the Avengers, using Tony's suits and Steve's shield, and also Hank's Ultron (which slowly makes more and more of the decisions for Zemo). From here you have some fun like they did with Dark Reign. Don't want to make a PG-13 The Punisher film? Make him the "Black Widow" of the Dark Avengers. That would garner some interest. Bullseye as Hawkeye. Etc. Have this highly dysfunctional (e.g. Frank and Bullseye trying to kill each other while on the same side) team legitimately save the world from some new threat.
THEN you have The Avengers 3, where Nick Fury resurfaces to stop Zemo's Ultron (who's really running the show at this point) from taking over the world and whatnot. Avenger vs Avenger action (without the mess that is Civil War) means a lot of hype. I'd prefer it if Ultron had a creepy father issues with Pym.