The guy I'm most looking forward to see in X3 is Daniel Cudmore's Colossus. Same kid who played him in X2. And I can't much think of a better Juggernaut than Vinnie Jones, although I expect to be disappointed by the lack of size vs. the comic book version. Oh well. I think Grammer will be fine in makeup as the Beast. And Ellen Page as Kitty Pryde is also good casting.
I'm looking forward to an expanded role for Colossus, too. I nearly stood up and cheered in X2 when he stepped into that doorway and armored up. He needs a bit more of a Russian accent, though -- he didn't really have one in his few lines in X2.
The most disappointing thing about the X movies has been their treatment of Cyclops. Few lines and only one good scene in each movie for the first X-man. Instead, he's the brunt of jokes (Backstreet Boys in his CD player? Come on!) or a scowling, jealous boyfriend/husband.