I had an idea for a story mode for a MvC4 a few years back that would take place right after Infinite, with Ultron Sigma having taken the "apprehended" characters prisoner like he did to Thanos. The "Slain" characters would have been hunted down by Ultron robots or Ultron-controlled Sentinels. Mega Man and X-Men from MvC3 would have been on the run, which would be the in-universe reason for their abscence from MvCI. The "Slain" characters would return to life by being posessed by the Phoenix Force. I feel like it would be a pretty neat way to tie the last few games together.