In all seriousness, the WWE title is already on the line. The match doesn't need anything else added to it. Besides, I'm almost positive that WM30 will be the conclusion of Rock/Cena story and I'm sure there will be an added gimmick next year to sell the end of the feud.
The only way it could get my interest is with Cena turning heel. Cena is already getting booed by 60% of the crowd, he would be over huge as a heel and taking the title off the Rock at WM would cement him as a mega heel.