i also think magneto returning to continue doing what xavier wanted would be an awesome idea and have thought of that since seeing the last stand.
as far as the villain, i think going ahead with the apocalypse idea and sinister, if they dont use him in first class, would be perfect for a 2-part movie. a new trilogy seems like they would need a lot of fresh new ideas but i think if they were to film a back to back 2-part movie where both parts are 2 hours would be a better choice. i dont kno what they would do about the sentinels, but itd be awesome to see a sentinel or 2 show up in first class and they use the one in the last stand just as a training exercise based off of past events for the new mutants.
however if they dont use a sentinel ever again then oh well. there would have to be a real reason to use it and i think they may have already missed that opportunity. stryker duznt have to have anything to do with the creation of the sentinels and after already seeing a potential one in x3 i dont think it would make any sense to see it in a future sequel. like i said, they might be better off using a sentinel in first class or if there is a sequel to that one then they could use it then.
if they are making a 2nd wolverine and they dont use omega red i would want to see him used as a potential villain seeing as how wolverine would most likely be in an x4. but i like the idea of having apocalypse and sinister for a 2-part movie the best. and they wouldnt even be the only villains, you would still need the 4 horsemen of the apocalypse. itd be cool to see if they could somehow use angel rogue and sabretooth as 3 of the 4 horsemen. it could give sabretooth his much needed closure and it could respark rogues powers and give her control over them