SilverKey said:

Could some scenes be rearranged to make Anakin's turn a bit more gradual?

Imply, but don't show Anakin killing the Sandpeople, and don't have him confess it to Padme. That might make him a bit more of complex character, and might add an extra layer to his quest against death (wanting to help out his clone wingman in the battle, hesitating to, and regretting killing Dooku, not wanting to leave Obi-Wan behind, trying so save Padme): he's doing it out of guilt.

Then in ROTS, Anakin watches Palpatine kill Windu, but doesn't actually take a part in it. He sides with Palpatine and is sent to Mustafar to kill the Separatists. The Separatists have been the bad guys the entire trilogy so Anakin killing them would still be his first dark deed as a Sith Lord, but still be somewhat ambiguous.Then the fight with Obi-Wan takes place and Anakin is injured.

When he wakes up, Palpatine tells him he has lost everything, and that's when the true Jedi hunt (Order 66 and the raid on the Temple start).Maybe those scenes could be the climax of the movie.

I'm not really sure how and if it would work, but I think it might make Anakin's turn a bit more gradual and layered.

I really like this idea. Vader would be suited when he raids the temple. His anger with himself after his injuries would make this scene more believable. Someone made a mock up of this, but I don't remember who. Seeing the suited Vader enter the temple would be a spine-chilling moment and be more reminiscent of what I thought of when Ben tells Luke of Vader hunting down the Jedi.