General Grievous
I have a theory linking Sifo Dias and General Grievous. We all know that GG is part alien and part machine and has the wherewithall to handle multiple lightsabres. The questions remain: how did he come accustomed to handling lightsabers, and if he is mostly machine, why doesn't the empire produce 1000's of GG.
Perhaps GG was Sifo Dias, a jedi who was presumed dead and then was "rebuilt" by the Palpatine the same way Vader will be. GG being an ex-jedi would explain his saber skill and his stategic intelligence. Assuming the only organic parts of GG body are his brain and possibly his eyes, it is possible that is almost all that was salvagable of his body when whatever ill fate fell him. That sort of damaged would explain why the jedi thought he was dead, and having an alien intelligence behind all that cybernetics would explain why the empire is not mass producing GG.
Remember the nervous look Mace and Yoda shared when discussing his name?