If there was an episode VII though, the whole Windu as the new Sith Lord would totally work. First off, there is no proof that he died. He could easily have survived that fall. Look at how easy Anakin and Obi were able to grab on to speeders passing by in AOTC. Also, Windu is very close to the dark side anyway with his Vapaad and everything. Plus, we have seen him several times stray from the Jedi ways and go for the kill shot when it was not necessary. He's arrogant, powerful, and would make a strong Sith Lord with another 25 years to train in the dark side between episodes III and VI.
Yup, I can see it now. Luke and Windu have to square off at the end of Episode VII. Jedi vs Sith all over again... 🙂