If we are talking about going back in time to alter the Episode 3 script (and therefore the final movie), I would say eliminate Greivous altogether, he was fucking pointless and served no purpose.
But if George said 'No, I really want there to be some kind of cyborg villain in Episode 3 who has 8 arms and shit then in that case I would say 'well let's make him Darth Maul back from the dead' - It ties the episodes together, saves us from the introduction of yet another character, and also suggests that Palpatine was dabbling in cybernetics before Vader. Greivous/Maul would be a prototype for Vader, in a way.
If we are talking about trying to make a fan-edit that converts greivous into a ressurrected Maul I would again say no - I don't think it's do-able.