I disagree with the “Vader should have turned earlier” idea. It just doesn’t “rhyme” well with the OT. In the OT, we had a definite set of core characters, and IMO the same should be the case with the PT. It would seem clumsy if the protagonist of the first 2/3rds of the trilogy became the antagonist in the last part. I suppose you could fix that by having Obi-wan be the main protagonist, but I’d rather have the prequels be Anakin’s story first and foremost.

In my ideal prequels, Ep1 would’ve had Anakin as a heroic protagonist. Ep2 would plant the seeds of darkness in him, becoming an antihero, and by Ep3 would be about him turning to the dark side, with the definitive turn at the climax. He may only become evil at the very end of the trilogy, but it’s a much more gradual progression that doesn’t feel forced.

One idea I like is that in Ep3, Anakin and Obi-wan should be rivals, who are either part of different factions fighting for the same general cause, or they’re on different sides but still respect each other until Anakin fully becomes Vader.