Re: What would you rather see next - FIRST CLASS 2 or X4?
If they HAVE to make an X-Men 4, and in the process they HAVE to bring back the characters who met certain fates in X-Men: The Last Stand, here's how I would personally do it:
-Xavier, Cyclops trapped in Astral Plane
Jean's powers trapped the minds of those she "killed" in the Astral Plane. From here, I would go with a story arc similar to that of X-Men Legends, and I would have Moira, on Muir Island, with the help of Forge, create a portal that would allow the X-Men to go into the Astral Plane to rescue Xavier and Cyclops. Moira would be involved with this because of Xavier trying to contact her through the brain dead patient. However, he wouldn't fully be able to return from the Astral Plane, and thus, Moira and Forge work on a portal to the Astral Plane, and the X-Men go into the Astral Plane to rescue Xavier and Cyclops. This arc leads for a potential Shadow King confrontation, as well as potential returns of Banshee (even as a cameo) and Emma Frost, depending on what story arc you want to take.
-Magneto is too powerful for the cure
X-Men: The Last Stand introduced the movie universe to the concept of mutant "classes". Magneto is established to be a Class 4 (Callisto: "There are 87 mutants in here, and none of them are above a Class 3, other than you two"; Xavier: "Jean is the only Class 5 mutant I've ever encountered"). I think that it would be pretty realistic to assume that Leech was not a Class 4 mutant. Therefore, my theory is that Magneto is able to regain his powers because the cure, extracted from Leech's DNA, was not powerful enough to permanently cure someone as powerful as Magneto. However, Rogue and Mystique could be established as Class 3 mutants, weak enough for the cure to be permanent.
An alternate theory, but would probably be much more cheesy to explain, would be that when Beast put the needles into Magneto, they never penetrated Magneto's costume, and the cure was never actually injected into his blood. Magneto's reaction on Alcatraz would be from the fact that he was in such shock over the thought of being cured, but he never actually was.
That said, I'd still prefer that they just continue on from X-Men: First Class if they absolutely need to continue the franchise. I prefer the characters of the main trilogy greatly over the characters of the First Class series, but their story has been told. It's over.
I think the best course now is to continue on with the Havok, Banshee, and Beast core of characters, slowly introducing some other characters from the comics that we haven't yet seen, and explore this story arc. If the X-Men: First Class series truly does go long enough, then we can tie it all together in the last movie with a shot of Xavier's new class of Cyclops, Jean, and Storm or something.
I wouldn't mind seeing Sentinels or the Mutant Massacre adapted in some way in First Class sequels though. In fact, I think a First Class era Mutant Massacre would be an awesome way to bring Gambit back into the storyline. William Stryker is essentially the Mr. Sinister of the X-Men movies, and Gambit has obviously been involved with Stryker in some way (his file on Stryker's computer in X2, as well as his entire arc in X-Men Origins: Wolverine), I think having him lead the Mutant Massacre under Stryker's command could be a pretty compelling storyline.