I actually have mentioned the idea before that Anakin could be corrupted already before Episode III, and we only find out at the end. References could be made in the opening crawl to a secret agent assassinating key Separatist leaders, drop references here and there, etc. Perhaps we could even see Anakin's slaughter of the leadership on Mustafar, but keep his face shrouded till we see the infamous "Sith eyes" shot. Only then do we realize that the secret agent has been Anakin all along. Perhaps some shots in Episode II could be edited to allude to him secretly signing up with Palpatine, though I'm not sure if he'd be signing up with a Sith Lord or simply with the Chancellor as a secret agent. In any case, I haven't thought hard enough about this to consider the plausibility and what shots to use, but it's idea I thought I should throw out there again.