I don't have any strong opinions about it, but I am interested in hearing yours.
Off the top of my head...
Ed Harris -- The Rock and History of Violence. Seems like his type of villain would not only be menacing, but more in-your-face and take charge. If he caught Bond, he'd shoot him, not give a long speech. I think he'd add a definite "real world" feel to the whole archvillain shtick, keeping in line with the supposed grittier feel of Casino Royale.
Benicio Del Toro -- would be interesting to see him "graduate" to the role of main villain.
Ken Watanabe -- since he really wasn't what he seemed to be in Batman Begins, a Bond movie would give him a chance to strut his stuff as a villain. Besides, it would get Bond back in Japan, which is something I definitely want to see!
Anthony Hopkins -- Well, one can dream, right?
Lee Tamahori -- Just kidding!

I wanted to suggest Gary Oldman, Alan Rickman, John Malkovich, Patrick Stewart and/or Willem Dafoe (who kinda already did it in EoN)-- but maybe those choices wouldn't seem "fresh," I dunno.