It would be quite nice. It could either be used to turn him heel, let him prove his moral values or even become the first cash in looser. Imagine if he won the briefcase and anounced on raw the night after that he would cash in at the next PPV and the champion wouldn't have another match. But what if the champion was Orton or Punk, Cena could realisticly loose and become the first cash-in looser, whcih would take the pressure of for the next guy.
In additoin I think it's stupid that it's always some up and comer who wins the briefcase. It seems unrealistic. Kane's victory was very nice since he had been in like a million MitB matches before. Also, why shouldn't the best win? The best being those who have held championships before.
I know that Cena hasn't been in a MitB match before, but one time has to be the first. the second point would work equally good with Orton or Sheamus or Big Show.