I feel like the one thing the prequels needed to do (and IMO it didn't do it nearly well enough) was to show how Anakin was personally responsible for the state of the galaxy in the OT. It should have shown that Palpatine's schemes, brilliant though they were, would have failed and he would have been defeated by the Jedi were it not for Anakin's betrayal. Without Anakin, there would have been no Death Stars, Star Destroyers or stormtroopers reigning terror over the galaxy and the whole plot of the OT would have been averted. I know there was that scene in ROTS where Anakin saves Palpatine from Mace, but I think it needed to be a deliberate decision on the part of Anakin rather than an impulsive one. I basically imagined the plot of ROTS being that the Jedi were putting together the clues and were on the verge of discovering Sidious's identity, when Anakin begins to sabotage their efforts from within, and by the time the Jedi realize he's a traitor it's already too late. The other thing that the PT needed to do was to expand on the "Obi-Wan once thought as you did" line in ROTJ. We needed to see Obi-Wan make an attempt to convince Anakin back to the light side, and we needed to see him almost succeed...but yeah, that didn't happen.