I got a better alternate timeline.
Suppose Grievous killed Obi Wan, and then proceded to another Sep controlled planet when order 66 came around? Surely, the kaleesh woulda' figured out what was going on, then proceded to kick Anakin's punk ass around on Mustafar (killing him outright instead of leaving him for the flames), and led an ongoing war against the "Empire" as it was being reorganized. Plus, with no news of Grievous's death to report, Windu would have stayed in the temple, an reported the news of the identity of Sidious to the other Jedi masters, giving them a pretty good warning not to trust the clones. One more master at the temple would have definitly put a dent in the 501st. I'm not sure if there were any jedi masters in the temple at the time...
Resident Spammer: feel free to torture, maim or simply ignore.