It's all up to the booking at this point. If they book the Royal Rumble well and continue to do so all the way till Mania, then I guess our concerns should diminish a little bit.
If Cena, Rollins, Orton and Cesaro can manage to get back ahead of schedule, which isn't likely (especially for Cena and Rollins), then that'd be great.
Clearing Daniel Bryan would be a wet dream, obviously, but Vince fears having Bryan back could ruin Reigns' overness, so let's not hold our breath for a DB return during Mania season. However, if he doesn't come back till Mania, then I have this feeling he might appear on the post-WrestleMania Raw episode. Time will only tell.
On the other hand, we have The Rock advertised to appear at the event, although it's still unclear if it'll be in a wrestling role or not.
But, given that they're gunning on making WM 32 the biggest WM ever, it wouldn't surprise me if Rocky wrestled on the show.
Now, if it'll be against Lesnar or HHH, that remains to be seen.
