I have to disagree with your last statement about last Mania. We had seen Batista vs Cena before, but it was 360 degrees different than what we got for Mania. Their Summerslam build up was a joke (they had the audacity to hype the match as "no hype needed") besides the fact that Batista probably lead us throught the build up with perhaps the best mic work in quite some time. The only thing bad about the match was that Batista made such a good work presenting himself as the big villain and Cena as the goody good underdog that it was obvious how the match was going to end it, but I can say Batista vs Cena was a bigger draw than anything WWE could put right now.
HBK vs Taker was just legendary. Seriously, it gets no bigger than that at Wrestlemania. Imma go out on a limb and say that no other match since Rock vs Stone Cold WM19 has captured what a Mania match is supposed to be quite like this one did, and every day HBK stays away from WWE, the match's legacy gets bigger.
Anyways, I don't know what WWE will do for Mania, but they certainly have tons of options. I have said it before, WWE doesn't like to do rematches for long running feuds at Wrestlemania, even if they do the most sense storyline wise, so I wouldn't expect HHH vs Sheamus nor Kane vs Taker for example.
Miz will be in the main event for sure, altough against who and for which brand really depends on what they do for John Cena.
In fact, this entire event depends on what John Cena will be doing. John Cena vs Randy Orton, in a face vs face megaclash is still an option, even if they decided to waste their first face vs face encounter now on Raw. If they do this, Miz will find himself in the midcard or challenging Edge on the SD side, who I have no doubt will enter Mania as the champion.
If they turn Cena heel, or even if they don't, they have him face Taker, I think we will see Miz actually DEFENDING the WWE Championship against Randy Orton.
HHH is another interrogant. He can easly sneak his way back into a title match. HHH vs Edge is always an option, and is one of the few one time only matches WWE still has. He could be facing Wade Barret in another undercard match, he could take Taker.
Y2J could as well be thrown in a title match. He could be back and get revenge on Orton. They could give us Miz vs Y2J in a midcard match.
I don't know, I truly don't know, and I like it that way. We still need to factor in the WCW factor WWE might want to implement. It's Atlanta, so Goldberg might get a match. Batista might get his thumb out of his ass and get back for a payday (and I wouldnt get mad if he returns cutting the same level of awesome promos he was). In what might shock the world, Kurt Angle might finally give up on TNA.
I am only sure we won't be seeing long running feuds ending at Mania, on the line of Cena vs Barret, Sheamus vs HHH, Kane vs Taker. Experience tells us so.