I apologise if this has already been discussed...

My biggest gripe with the whole movie is Anakin's turn and how he willingly slays a group of kids as his first act after the turn. If somehow you could change it so the stormtroopers attack the Jedi Temple and kill the kids while Anakin goes to kill the Trade Federation on Mustafar it would make it a lot more plausible: and in the process it would cement his Dark Side turn as per Sidious' wish (killing unarmed, helpless idiots). Bottom line, I just do not buy that Anakin would kill those kids in cold blood (and no it's not like killing the Sand People, which was done in blind rage).

For me this is the moment that kills the film, it's Lucas' biggest screw up in the prequels, and if you could somehow make that change it might make his turn believable.