If Sinister was included as Dr. Essex before he became Mr. Sinister they would have to change his origin quite a bit. Though they have done that with more than a few characters already.
If this does happen then how would you see him becoming Mr. Sinister? I could see it working if he was a lab scientist as said in the rumours because it could be explained that he was fascinated by the mutant condition working there and with Angel and his obsession would tie in nicely with Archangel.
As for possible actors, I don't really want to see a known face. Though I like some of the suggestions, I just don't see them working, especially Walken. Don't get me wrong, I'm a great fan of his work and I think it would be good on screen but it would seem too out of touch with what has gone before in these films. The casting should be less about Hollywood greats and just about who can deliver the right peromance.