Not sure about Sean Bean as Sabretooth. He seems not strong enough to do the job. I guess if he keeps his Brittish accent shut, then he might be able to do it. I'm a fan of Sean Bean, and i agree he's great in playing dark and gritty characters, but I don't see him as a Sabretooth.
As far as Wolverine, I would never replace Hugh Jackman. He did a great job in all of them and even if there are other actors who could do the job, changing Wolverine as a character (which replacing an actor wouls be) would probably be a mess to the universe.
Elisha Cuthbert for Emma Frost has always been my first pic as well! Perfect choice!
Taylor Kitch did a decent job on Gambit in my opinion. Tho I agree the character was just written poorly.
For me the return of Deathstrike (how impossible it might be, but so many have returned from the death in the comics) and Nightcrawler would be perfect. Kelly Hu did a great job as Deathstrike, she looked dangerous and the scene where Striker's poison (I can't think of the name) is done working is brilliantly done in my opinion.
As the writer I am, I've always tried to work on an idea for the next X-men movie. And I thought that Emma Frost should definitely be in it, and I think Magneto's part should definitely be done (Ian McKellen is great, but we need new villains). I'd say bring in Mr. Sinister and Apocalypse (how about Dalip Singh). And in my idea I used some of the less popular mutants, Mimic and Infectia. I think they really could be interesting movie characters.