Maul-Grievous is a damn good idea. If they had used that as his backstory in RotS it would have added a hell of a lot more depth to his character. For starters it would give Darth Maul some of a connection to Darth Vader. Not only would he be a mechanical monster with asthma, but he also would have been cast aside in favor of a more powerful apprentice.
Plus it would have given Lucas a good reason to have him show up in AotC ..... oh how I hate that movie.