Whedon wanted the Maximoff twins because Wanda was the only one who could beat Ultron and her powers were something no one has really seen yet.
I seriously doubt they will kill anyone of the big names off. War Machine won't be killed off since he and Falcon are the only black characters so far, you know someone will stir negative controversy accusing them of racism. Rhodey is a good character they just should have cast a badass instead of nice guy Howard and gentlemanly Cheadle. Marvel has invested a lot into building these characters and this universe and I don't see them casting them aside easily. Renner is supposedly getting a bigger part in Cap 3. I don't see Evans being written out anytime soon. The last film made a huge subplot of his love life, they aren't going to get him laid then kill him. I don't know how confused the movie audience will get at someone else (not Rogers) as Cap since not everyone is as **** as comic fans. If Bucky takes over I would imagine Evans would have to be right alongside him in the same film doing some other job.
I can see after A3, Thor may be off world to deal with cosmic post Thanos stuff. Downey will likely reduce his part to cameos but I can't see him closing the door since there are so many creative possibilities.
Hulk is always a wild card since as Hulk he doesn't work and play well with others so he may not always be an avenger. It really won't be hard to explain absences and Marvel has shown that they just want this universe to keep growing so they won't see a need to thin it out or shrink it. I think the biggest problem will be finding bigger and bigger threats for future team movies. Unless they battle the Celestials lol,
Especially when they keep killing the villains.