^ Totally agree that Sinister could hold his own as a main villain. Especially if they go back in time to show Nathaniel Essex's origin. That would be pretty cool!
Apocalypse needs to be an X-Men villain. Yes some of the X-Force cast could be in the story, but he needs to be the villain of the X-Men. But the great thing with Apocalypse is that in Age of Apocalypse anything is possible. Any dead character could be back. Just cause Cyclops is dead doesn't mean they couldn't bring him back.
But let's analyze each of the main characters in AOA:
Legion: Do we HAVE to have Legion be the one to kill Xavier? No, but it would be cool. I've seen it done (I believe in a cartoon) where Wolverine (or was it Bishop) comes back from the past and it is now the Age of Apocalypse. Could be done either way, though having Legion (or someone else) go back to the past again would enable the First Class cast (some of them) to be used again.
Bishop: He's the only one that knows that the timeline was changed. We will have Bishop in DOFP, so he's good to go.
Magneto: Could possibly use Fassbender as the Magneto in the AOA. Once the past changes, does what year it is really matter? Guess it does a bit since these X-Men movie timelines are so messed up. Either way Mags leads the X-Men against Apocalypse.
Astonishing X-Men (Rogue, Sabertooth, Blink, Wild Child, Morph and Sunfore). This group could be changed up some, but effectively Mags sends a team to stop the cullings.
Amazing X-Men (Quicksilver, Storm, Dazzler, Banshee, Iceman and Exodus). Same thing, these characters can be changed up to be just part of Mags' X-Men.
Weapon X and Jean: Very important characters (and popular to the GA). The relationship between Wolvie and Jean are important.
Factor X: Cyclops is important. I would normally say his brother is as well, but in our X-Men movies Havok won't work. What is important is that Cyclops is a "bad guy", but he is really helping people escape Beast's pens and he is then helped by Jean.
Dark Beast is important as a minor villain.
X-Man is probably not necessary.
I know many of you think that Angel is important, but why?
He could still be made into Archangel by Sinister in a future movie. I know, he's supposed to be the Horseman Death, and I guess this could be worked into the movie...
So really, who do we need?
And then some others (Blink, Storm, Etc.)
So why is Cable so important for AOA?????
I know that in a different story it was Apocalypse that infects Nathan, but did that happen in AOA? Couldn't that happen by Sinister? Guess it could happen in AOA. Still don't see why Cable is needed.