If we're completely rewriting the PT, I honestly think that Grievous should stay. By all means, I agree Dooku should be a villian for all three movies, but I personally think Grievous served as a very nice transition to Vader, and could have been a great character, but wasn't used as well as he could have been. It would be a very nice touch, in my opinion, to have him be someone who is just always there, maybe show him having a lightsaber collection, and only hinting at his ability to use them until Episode III, in which he could go into some sort of slaughterfest (depending on the scenario involving Jedi). And by all means, he should NOT be killed by a freakin blaster...