He's retired but it would be interesting if prime Nobuhiko Takada would face Cena in a Shoot match, seeing as how Lou Thesz claimed Takada as the true professional wrestler in the early to mid 90's.
Takada got beatby anybody and everybody in MMA. I think Cena would wreck him. His wins in MMA are all widely recognised as works. His fight against Cro Cop is only a draw because it was under special rules which stated if time expired there would be no judges decision. Hence he just welt ultra defensive. Watch his fight with Mike Bernardo, all he does is stand off the whole fight.