Hugh Jackman is a given, of course, but I'll be interested to see how they plan to fit others in. It's worth noting that the two major Apocalypse stories in the comics were both kicked off because of time travel. He's a 5000 year old mutant who was in hibernation, waiting until the world was stronger when others like him emerged, so that he could test it, conquer it, and build a world where only the strongest survived. In two of his major storylines, he was awakened due to the arrival of powerful mutants from the future, which touched off a stronger use of worldwide mutant power. That's what made me wonder if Logan's trip to the past is what kicked him awake earlier than he was meant to awaken. But maybe they're going to keep using time travel and send someone else back from the future somehow to awaken Apocalypse in the 1980s. If there's a future section showing how and why that character goes back in time, even if that character isn't one of the original cast (in the comics, it's initially Cable who goes back and accidentally awakens Apocalypse by his presence), then it would be an excuse to use some of the original cast again.

Maybe the story will start in the future with a world under attack by Apocalypse, and someone goes back to prevent his rise, and accidentally awakens him in the 1980s when things will be much worse and the First Class team has to fight him instead with the help of the time traveler. Except that would smell an awful lot like what they just did in Days of Future Past, so I don't like it. I guess we'll see.