Hell, I don't think any match should be main eventing over Cena/Taker. It is one of those matches at WM that will have a huge impact after it is all said and done.
Only if Cena turns heel. Otherwise what's the point? Taker goes over and then retires or Cena goes over and everything is back to same old shit the next night.
If Cena is turning heel, then that's your main event. Otherwise Rock/Brock might as well close out the show.