I dont know whether you mean Wrestlemania 27 or any Wrestlemania in the future, so I'll come up with matches for any Wrestlemania in the future.
John Morrison vs. R-Truth - Both men have been an on & off team, there was tension between the two during their feud with Nexus before they went back to being eventually friends, all in all, they're like hints to me that WWE are very slowly building up a match between these two "best friends" one day and that day should be Wrestlemania and why not? Truth is a black Morrison, it would be a good matchup especially for a title.
Undertaker vs. HHH - Simply because of the HBK storyline from last year and because these two havent faced off in a one on one match in a long time and at this point in time with the two being even bigger mega-stars now from the last time these two faced off, it only makes sense it happens at Mania.
Taker vs. Cena - The two tops dogs of the company and with Cena being the first top dog, a lot of people will be soiling themselves complaining about the possiblity Cena will end Taker's streak cause they're just that damn stupid to believe that, so it should be done to put off fans from believing Taker will win.
Randy Orton vs. Kane - If Orton still has the Viper gimmick then these two should face off at Mania simply because they both have dark gimmicks and are World Champs at this moment. So if neither men are champion at WM, it's only right these two fight each other especially as we never seen em face off before instead of putting Kane & Orton with some midcarder to face and it wouldnt be too obvious that Orton would win whereas if Orton faced someone like Jericho which I was originally pulling for, Orton would win simply because he's the Austin of this generation.
Edge vs. Christian - Both are now much bigger than the last time they faced off. I was also gonna mention it being a Brother vs. Brother match but then I remembered them claiming to be "best friends" instead of brothers, so I'm guessing WWE has scrapped the storyline of them being brothers.
Melina vs. Gail Kim - Why has this not even been done yet? Melina is the new Trish Stratus of this era and Gail Kim is the one & only entertaining diva today that hasnt faced Melina yet. These two could put on a good match if put together. It would be a like a Trish Stratus vs. Lita match if you ask me which is the two should be put together at Mania not any PPV.
The following matches arent needed for a WM but it would be nice to see. Sheamus vs. Big Show or Booker T or Goldberg, Wade Barrett vs. Big Show, Chris Jericho vs. Daniel Bryan, Rey Mysterio vs. Kaval.