That would be a HUGE surprise. There's no chance they'd throw Reigns into that, he'd get booed out of the building.
Reigns vs Cena makes the most sense (not necessarily for the title), with Bryan vs Brock (less likely to be the title match). Rollins vs Orton could be a great main event, and would be the closest thing to a shock, but I don't see it being a title match. Ambrose has no spot on the Mania card as I see it right now (neither do plenty of other guys so no need to worry there).
Reigns vs Cena
Bryan vs Brock
Rollins vs Orton
Undertaker vs Wyatt
Sting vs HHH
Look pretty likely at the moment. I am interested in what happens with Rollins, he's the key if they decide to change things around.