They're saving Reigns V. Lesnar for WrestleMania
This is obviously no secret and WWE went ahead and made sure to plant those seeds the very next night after Wrestlemania via Paul Heyman's promo. I've already talked about it myself and as much as most of us are not fond of the idea, unfortunately WWE (well...Vince) is already locked in as this being the main event in New Orleans with Roman of course going over, slaying the beast and blah blah blah.
Assumimg the above is true, Roman Reigns should be the least likely amongst the candidates to win and can be excluded relatively easy. As for the rest of the participants, I would personally like to see Samoe Joe come out the winner but being he's a heel and being Brock is also considered a heel, this option is probably excluded just as easy since WWE usually like the face versus heel aspect. In Seth's case, as already noted above, it's a "been there, done that" deel so if WWE are looking to give us something fresh, I'm axing this option as well so that being said, I personally see it coming down to either Fin Balor or Bray Wyatt.
In Balor's case, I understand there's a lot of talk about the size difference between him and Brock and how "believable" can it be that he can actually win. To this I call dogshit cause it's been proven time and again that size doesn't really matter; quite frankly, it's a rather simple storyline if WWE were to proceed, making it a classic David vs. Goliath type of thing. My personal "question mark", if you will, in this scenario is this promo we got last Monday on RAW. Don't get me wrong, Heyman putting Balor over was great and all but if Balor were to win at Extreme Rules, they kind of made very predictable already, don't you think? Granted, it wouldn't be the first time WWE got extremely predictable and there's certainly no doubt that Balor vs. Lesnar is a money match but after Monday I'm really second guessing this scenario.
That being said, as of now I am leaning towards Bray Wyatt being the one to eventually be facing Brock Lesnar. This is something that WWE were already looking to do in the past but never did and as I noted some time ago (well, a couple of years back) when predicting this specific match for Wrestlemania, the promos alone between Bray and Paul would be a treat for us fans. Furthermore, Bray isn't exactly considered a heel per say and he's one guy that has practically become "numb" to losing, meaning it doesn't even matter. We're already used to see him lose (eater of pins, is it?) so another loss here, especially against Brock (whom apparently has to retain) wouldn't really hurt him in what should probably be a one time deal anyway. After this and assuming Braun will return soon enough, WWE can go back to the Bray/Fin storyline that was initially the plan...
All in all, I wouldn't consider Bray the most popular choice, but I surely consider him the "safest" option...