Agree about Kitty. She should be part of this storyline, the same as the source material.
I should say Wolverine too, for the same reason, but I wont be mad with his absence
About future characters, I voted for Storm, Kitty, Collosus and Magneto (I dont mind Xavier out in the future)
Id keep Riptide and Angel, but Avalanche would be a great addition, specially been part on the arc of the comics.
Im not sure about Mystique and Azazel sleeping together on the second movie, mainly if its set in early 60's yet. If Fox does more than 4 movies, I think the third movie would be the right moment. If not, this is the moment, of course.
And about the future presence on the movie, I think a prologue, more short parts during the film wouldnt be a bad idea. Not just a prologue and a final scene. But the main set should be the past, in my opinion. I want the First Class cast to be the main characters, and the original actors as large cameos/really secondary roles.