I've always felt that ROTS's changing of the Galactic Republic into the Galactic Empire always felt too much like flipping a light switch. Sure there was political build-up, but on the whole I thought the events felt too forced and sudden.

Personally, my prequel re-write has the Republic dissolve several years before the formation of the Empire. After a clone fleet surprises Coruscant by bombarding the seat of government with orbital fire, the Republic fractures into my disorganized warlord factions.

Ostensibly the Republic remnant factions are allied with each other, but they have little trust for each other. For much of the Clone Wars, the clones rule the galaxy. It is only when the strong-man political leader and military officer Admiral Palpatine steps into the forefront that this rabble of fleets and armies is united into a galaxy-spanning Empire.