So I have a contribution to the flash forward storytelling idea. Anakin's first Jedi massacre of course is at the Jedi Temple. Obi-wan learns the truth about him and goes to face him on Mustafar as he loses/becomes the version of Vader we all know and love. Ben thinks he's dead, they split up the kids, and so forth). But unknown to most, Vader survived, and then goes on his rampage killing Jedi. He could be shown as the one giving Order 66 in place of the Emperor. He could shoot down the Jedi Master in the starfighter and on the speeder bike. Some new footage would be necessary, but perhaps he could even be shown dueling some random Jedi. It would require decent costumes and makeup, but people make lightsaber duel videos all the time. Then in the end we'd see Vader and the Emperor on the bridge of a Star Destroyer overlooking the construction of the Death Star. This has the advantage of not appearing so abrupt an ending and therefore allowing for a better transition from democratic Republic to despotic Empire.