This screams Iron Man 2 all over again, which was, imo, the worst Marvel movie post-well....Iron Man.
There was too much going on...Whiplash, Nick Fury, Black Widow, Justin Hammer, War Machine...even the action was not all that.
This movies does have the potential to be a mess, in a new era with a new tone (probably) and more differences than usual for a sequel. That goes especially if it also uses flashbacks heavily and doesn't use the characters wisely, but that's the same as many movie really.
That was the problem with IM2, I agree with posters upthread who said it was full of characters doing not much for most of the movie, including two of the most interesting storylines, Vanko and War Machine. Hopefully if BW is in this it will be in her Avengers persona + WS and not her pseudo "love interest"/hanger on mode.
The Avengers had a dozen or so plot threads that felt satisfying because Whedon was able to use the characters wisely and make every moment count. That's where the Russo brothers might prove their worth as well. As television producers/directors they juggle a lot of characters and longterm story threads and Community in particular can be very non linear, jumping all over the place.