Yep, Cena/Styles for Wrestlemania is written on the wall. In my opinion, the Rumble winner will either be Cena or Balor, and I'm not sure how ready Balor is to return. The Rumble will be 5 months from the time they said 4-6 months. A returning Balor winning would be a huge pop, and then I assume you'd get a Smackdown Elimination Chamber to determine their #1 contender.
I don't think Cena will win, simply because it's something the fans would revolt against. The same goes for Reigns winning. I just can't see them trying that again after the crowd shit so hard on it the last two times. I have to imagine Vince is at least somewhat embarrassed when a PPV ends with a face getting booed out of the building, and the brand split basically gives the WWE the perfect excuse to do a little fan service.
The real question is what happens if Balor isn't ready. I agree he's a top candidate to win the Rumble if he can make it back in time. I also think that Triple H will be pushing for a Balor/Owens match at Wrestlemania given how successful their title program was in NXT. Triple H looks after his guys, and Owens and Balor are definitely two of them.
But if Balor can't get back for the Rumble, maybe they'll finally do some real wish fulfillment and go with Zayn. Yeah, it'd be kind of soon to rehash Zayn/Owens, and they just fought on Raw, but Zayn/Owens at Wrestlemania could lead to their best match ever. You know they'd both go all out to try to steal the show, and Zayn going over at 'Mania in front of a huge smark crowd would be amazing to watch. Him winning the Rumble would undoubtedly get a huge pop too.
Beyond that, not sure where they'd go with either brand. Rollins will likely be feuding with Triple H at Wrestlemania, so he's probably not winning the Rumble. In fact, I wouldn't be surprised if Triple H returns at the Rumble and causes Rollins to be eliminated (or eliminates him directly), which would only add fuel to their rivalry. He'd be a top candidate otherwise.
The WWE will probably just have Reigns win again though, because Vince hates us.
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Also, unrelated, but I watched TLC on Sunday night while the girlfriend was over. My favorite part of the night was when they first showed Dean Ambrose and she commented, "He looks like he just crawled out of a sewer." That had me fucking dying because it was so true.