Sting vs HHH, Orton vs Rollins, and Cena vs Rusev have been rumored for months and it's pretty evident they're all happening. The only rumored plans I've heard for Bray Wyatt is that he's scheduled to face the Undertaker, but that all hinges on whether Taker works Mania this year or not. (If you've seen any recent pictures of him, he already looks like he has one foot in the grave for real, so... I'd rather he didn't.)
Lesnar vs Reigns is now official, although the annoying backlash over Daniel Bryan losing the Rumble may force their hand again where they'll feel obligated to insert him into the title match and make it a triple-threat.
The Usos vs The Ascension for the Tag Team Titles, maybe Nikki vs Brie for the Diva's Title, maybe a multi-man (ladder?) match for the Intercontinental Championship (Barrett, Ambrose, Sheamus, Ziggler, etc.), etc. People are counting on seeing Miz vs Sandow, but the Wrestlemania card this year is gonna be stacked and I really don't see much chance of them fitting that match on the show. Save it for later, I say. The Andre The Giant Memorial Battle Royal will probably be the annual pre-show match for every year from here on out.
The match limit for the main card the past several years has been eight. Last year there were only seven matches. So one or two of these will most certainly end up on the pre-show.