A lot of talk about Carol Danvers on here but how about introducing her through an Iron Man movie instead? I mean they have a deep history with each other...
As for calls for She-Hulk I'm not really sure how that'd fit, because they have said there are no immediate plans for a Hulk sequel and wouldn't that be the natural route for her?
I agree with the need for Hank and Janet, and yeah Ultron would be awesome!!!!! They need to sort out a movie for them asap in that case then because I'd actually be kinda pissed if they just threw them into an Avengers sequel like they're just fringe characters. If they could sort them out, add in Pietro, Wanda and Vision and I'd be going crrrrrrrazzzzzy
Hawkeye always had a thing for Wanda and that'd naturally be easy to give him more screen time too as he was underused in the first Avengers. Then Wanda for Vision... I'd have no beef with given Black Widow a background role either tbh... Thor could be off racing Thanos through the other realms (in his movies) making the challenge of Ultron even harder
Even if they went toe to toe with Ultron and then discovered the Vision at the end (or after the credits xD) that'd be great