They're a bit stuck really, usually the title match with the biggest stars in would go last but there's no way they're putting on a sub 5 min Wrestlemania main event, and they're not stupid enough to have Goldberg and Lesnar do any more than that.
Which leaves Orton/Wyatt but thats not a very good WM main event either. It's a decent-ish mid card match but if they put that on last it will go down like a lead balloon once the bell rings and Orton locks on his first chinlock.
So that then leaves Reigns/Taker and HHH/Rollins. Normally you'd have a face win the main event but Reigns being booed out of the building as the show ends is not how they'd want WM to finish. So HHH/Rollins it is. Maybe. That's not exactly been booked great and Rollins is far from a great babyface, and if they go on last HHH will probably win anyway.
So who knows. Maybe Taker beats Reigns, Reigns turns heel but Taker fights back to end the show.