Let's see:
fiction wise, we've had Lex Luthor from Superman/Justice League as president.(which didnt turn out so well)
If shadowrun manifested at least partially, we could have the dragon Dunkelzahn (Darktooth) as president (at least till that astral horror...incident...)
Wasn't Britannia from Code Geass effectively the american colony that grew into a world empire? I wouldn't be surprised at Lelouch or one of the other vi britannia family members taking a stab at it.
One question to ask though:
Who, in their (in)sane mind would WANT to govern the US in a setting/situation like this?