I think it's safe to assume that Brock's just closing out whatever business he has left at this point. As in, fulfilling his contract by showing up and not much else. Go out there, make the appearance, do the job, get paid, go back home to hunting and screwing Sable.
On the other side of the coin, you have Roman Reigns, who's probably gonna get booed out of the building, and the whole thing is gonna be far more awkward than Brock's other match with Goldberg. At least Goldberg had a following, no matter how hostile the crowd at 20 was toward him. Roman's got, what, a very small fanbase of women, children and Vince McMahon?
Next consider the drama from this past week with Brock walking out on a scheduled appearance and later showing up at a UFC event. Wouldn't be a big deal except he's rumored to be rejoining that org. It isn't like Taker showing up. It's a clear message and shot fired towards Vince and co.
So, if they don't outright take the title off of Brock and remove him from the event, change the main event and go another direction, let's just say that if it were me running the E, I would be a little paranoid at this point that my champion would sandbag the final match on the biggest show of the show. That's assuming he shows to begin with.
So, assuming he stays as professional as needed and shows up with the title to do the business, a middle-of-show placement would be good strategic planning in case the match absolutely flops or he flakes out mid-match and says to hell with it, pin me, pay me, I'm out.
Do you think, at this point, it would be better to close out with another advertised match?
– Sting, on the other hand, has been one of, if not the THE most long-awaited arrival ever in the E and his story with H is the longest running right now. Definitely a main event in its own right.
– Taker and Wyatt, well, there's that stupid rumor that they want to put the match high in the card so it happens after dark. It's Taker, so what the hell, would it hurt for him to main? Bray's superstrong right now, so like Sting/HHH, it's a main in its own right.
– OR, call the same audible, get Brock/Roman out of the way early and then have Roman pull double duty against Bryan or Rollins in the final match of the night? It would guarantee a good show-closing match. WWE's not really behind Bryan, that much is obvious, so a second main-event of Roman versus Rollins would be an excellent show closer, as well as make a ton of sense. Seth's been the top heel for the last year, Roman's the next big thing and they have a ton of history.
None of the other scheduled matches mentioned should main this Mania, but they're bigger on paper on paper than what's advertised in the main.
Again, I feel that finishing the wrestling year with Roman defending against Rollins would make a million times more sense than any other match, for multiple reasons (quality, prior history, etc). Roman wins the title, Brock gets wished well in future endeavors, show goes on before closing out with Roman and Rollins. Roman wins, Vince is happy and people are satisfied with a decent main. (Rollins can always have a rematch later.)
– Brock's a man on a mission, and while I can't fault him at all, I wouldn't trust him at this point to do anymore than simply show up.
– there's no hope for Roman/Brock anyway, so get it out of the way, mid-show.
– if not one of the other huge matches being the show closer, have Roman defend against Rollins, if not Bryan.
– Roman and Rollins makes way too much sense anyway, so while it would make sense to be the main of Mania, it makes too much sense for WWE to book it.