Well Q is a staple of the Bond mythos... I think somewhere along the road they need a new one.
I don't think there was anything THAT unique about Llewelyn's performance that the character should never be used again... I mean, he did a great job, but that'd just be a shame.
John Cleese likely won't return (there was a falling-out between him and producers). And frankly, I think he'd be out of place in the CraigBond world, since things aren't based around corny dialogue and jokes.
Q, when cast for BOND 2.2, needs to be different than we've seen him before. The old, grumpy guy was done perfectly by Desmond Llewellyn, and anybody who tries that is going to pale in comparison. Somebody needs to step in and reinvent it.
Alan Rickman would rule. he plays that kind of character so well, him annoyingly bantering with bond would rock