The secret to a great WrestleMania is a solid opener. It sets the mood and tone for the night to come and it's very important. Let's take a look at the past 5 WrestleMania openers.
WrestleMania 27: Edge vs Alberto Del Rio - World Heavyweight Championship
WrestleMania 26: ShowMiz vs. JoMo and Truth - Unified Tag Team Championships
WrestleMania 25: Money in the Bank
WrestleMania 24: JBL vs. Finlay - Belfast Brawl
WrestleMania 23: Money in the Bank
Now out of those five, the ones that stand out to me are both Money in the Banks, and the World Heavyweight Championship Match... Obviously Money in the Bank won't open up this year considering there is no MiTB match, but could the World Heavyweight Championship Match open again? Probably not. It may not have the greatest stars, but it promises to be a very decent match. I would say the World Heavyweight Championship Match will be about 3rd or 4th.
So then what opens Mania? The End of an Era match. That's right the Hell in a Cell match between Triple H and the Undertaker will open up WrestleMania. If this match goes late in the card like 6th or 7th, it's going to take away from every match after that. And say it does go 6th, it would take away from Punk/Jericho and Cena/Rock, and the WWE doesn't want that to happen. It needs to go early in the card, and in my opinion it would make the most sense for it to go first, and then follow up with Big Show/Cody Rhodes and Kane/Orton, matches that don't have much importance.