#2 The Rock will appear but Batista won't

The WWE's recent deal with Fox has seen the company secure a reported 1.2 billion dollars, and if that is the case, then they are going to want to splash some of that cash and bring in some very big names for next year's WrestleMania.
Former WWE champion, The Rock, has been rumoured to win next year's Royal Rumble match and if that is the case, a big money match with one of either Brock Lesnar or Roman Reigns is likely to be on the cards.
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If The Rock does decide to lace up his boots at next year's event, then it seems highly unlikely, given his massive universal appeal, that he won't be in the main event of the show.
The other rumoured big name for the event is former Evolution member, Batista. Batista is currently scheduled to make an appearance at the upcoming SmackDown 1,000 episode alongside his former stablemates, Ric Flair, Randy Orton, and Triple H. While that does imply he has a good working relationship with the WWE, and that Disney and Marvel are willing to allow the star to work for his former promotion, it doesn't mean he will be working at WrestleMania.
The biggest stumbling block at WrestleMania is likely to be the release of the as yet untitled fourth Avengers movie, which is due for release on April 26th, just a few weeks after WrestleMania.
While Batista's current relationship with Marvel is a little bit icy following the firing of Guardians of the Galaxy director, James Gunn, the promotion campaign for the film will be in full swing come WrestleMania time, and it seems unlikely the studio would risk Batista working a full match at the event.