The prequels should have been about the clone wars and Bothan spies.

Obi-Wan should have been revealed as OB-1 (an original cloning source).

Luke and Leia's birth should have been the result of a rape during the war.

"The best star pilot in the galaxy" should have been a sub-plot, Red Baron like.

"A good friend" should have shared ideology at some level. More tangibly demonstrated. Remembering that OB-1 could himself be a clone.

The Mandalorian Warriors could have had an outing as a specialist clone commando unit, or whatever.

Darth Vader should've been hunting Jedi to extinction in the second act, perhaps sparing OB-1's life in a moment of near-redemption - or perhaps killing OB-2 thinking he'd got OB-1 instead.

There's so much it could have been based on the slight information we have from the OT, it's such a pity Lucas wrote the scripts and didn't seem to get past the first draft.