Don't get me wrong, Keith Szarabajka did a good job as the voice of the Didact in Halo 4. A shame we didn't get much more from him, but oh well. Anyway, had Keith not voiced the Didact, who do you think would've done a good job in the role?
Personally, I think Jim Cummings would be cool, just watch his performance as Robotnik in Sonic SatAM and tell me he isn't menacing. Also, though he is dead, I think Orson Welles would've been perfect, his role as Unicron in the 80's Transformers movie was terrifying.
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Nicholas Cage.