I think First Class is an excellent movie, but you've got to wonder how they will push forward with sequels to this and market them. They're in a tough place right now, trying to play this as both a prequel and a franchise reboot without really committing to either one. The decision to act like the previous movies mattered, but also set this series in the 1960s, puts heavy restrictions on where they can go with this.
Vaughn has already said that he wants to add only one major character to any First Class sequel. That is a valid artistic choice, and I'd rather a movie focus on character than try to jam in as many new mutants as possible. But can they really make an X-Men series without all the famous and iconic members of the team? No Cyclops, Jean Grey, or Storm. And no Wolverine either, who is clearly this franchise's star in the eyes of the mainstream.
Would Fox see such a series as worth it, financially? Would they mandate that the well-known characters be used, or even abandon this prequel series idea and continue on into the future?
Personally, I think I'm about ready for this franchise to come to an end. Not due to lack of quality - I feel we've gotten 5 great movies out of the deal. But, each movie that gets introduced causes more and more continuity issues with the others, and it's not going to take too many more sequels for this franchise to cluster f--- itself into obscurity as it further meddles up the continuity by directors who "don't want to be beholden" to the previous movies, but do anyways.
Except for a couple villains, like Apocalypse, Mr. Sinister, and Sentinels, pretty much all of the major characters have been portrayed in one way or another. While there are still many big time stories to tell,
X-Men: The Last Stand kind of prohibits the series from moving forward creatively, and if they do, it's going to be a situation of the franchise spreading itself too thin (either undoing major events that will make audiences roll their eyes, or proceeding forward without major, core players of the lore), and many of those stories are stories that audiences won't take seriously on the big screen anyways.
Unfortunately for me, I'm pretty much backed into a corner. As far as I know, in order to keep the X-Men rights, Fox needs to keep making movies. So they will keep making movies to hold onto the rights. And if they don't, then X-Men reverts back to Marvel, who will give it the unholy reboot treatment, and probably cluster f--- it into the Marvel Universe and having the X-Men fighting alongside Captain America...


First Class gave some dignity back to the X-Men movie series. I'd love to just see the franchise end on a high note, and let it rest.