I think people are being a bit short sited, is an inter-brand match really out of the question? Especially given the current weak state of Smackdown's main event scene and the strength of Raw's would it be so far fetched for an inter-brand match to happen? After all last years two main events were both inter-brand matches (Batista was on SD until his feud w/ Cena started).
There is also the possibility that Edge may not even walk into Mania as champ, there is a lot of time between not and Mania, who is to say Edge doesn't piss off the wrong guy or that WWE officials decide Barrett vs Taker should be Title vs. Streak. There are still a lot of things that could happen.
Lets just assume he is still champ though, who should his opponent be? Well my first thought would be;
HHH: Lets see, they have never feuded before in a major way and they have barely ever wrestled each other, this is one of the few main event level feuds that hasn't been seen a million times before and it screams Mania main event. I know WWE seems to be keen on pushing young guys strong but if they want WM to draw big this is the match for it.
The only issue is HHH needs to finish off his feud with Sheamus, no problem though. Lets assume he returns (and wins) the Royal Rumble, in the process he can eliminate Sheamus and reignite their feud. They can then feud for a month leading up to their big blow off match at Elimination Chamber, hey not like it hasn't happened before, an expected Mania main event happening the month before that is (*cough* Angle/Taker and HHH/Austin *cough*) This way HHH is cleared up to feud with Edge leading into WM.
If WWE decides not to go that route there are plenty of other good options in the likes of Christian, Punk, Miz, Alberto or even a freshly heel John Morrison. All of those would make for good feuds and matches but HHH is who I would go with. Also baring Punk a lot of those guys are very fresh to the main event scene, I know Vince is keen on pushing these young heels hard but I can't see the likes of a Del Rio or Ziggler in a Wrestlemania title match so early in their careers.