In the this video, Dave Filoni explains that Darth Revan was very nearly included in the recent Clone Wars Episode Ghosts of Mortis. He (or she?) was modeled and the scene was animated in rough form before they decided not to use him. I asked a few weeks ago why the cartoon was so bad, and this video does a good job of explaining it. The writers and Lucas apparently spend an enormous amount of energy agonizing over how the force "works," and not remotely enough time on the basics of good storytelling.
*Buy Darth Revan toys at eBay.