Sinister for First Class 2, Sentinels for X4.
I'd base a forth film heavily on Days Of Future Past. There could be a fantastic, meta-textual need to undo certain events. I switch Kitty for Rogue, sending the latter back in time to before the cure situation came about. She would reveal how everything became hostile after the cure "wore off" and riots began. The X-Men (Xavier,Cyclops,Storm,Rogue,NightCrawler & Ice-Man) try to prevent the nightmare future from happening.
It could patch over X3, keeping it canon but demoting it to an alternate future. You could keep the original cast, and marginalise Wolverine for a while (he was still drifting before X3) but keep him as a big figure in the Sentinel future scenes to keep everyone happy.