#1 The Fiend vs Roman Reigns
If the 'Big Dog' becomes the Universal Champion at the 'Grandest Stage of Them All', the Fiend vs Roman Reigns is the likeliest thing that WWE would consider after WrestleMania.
Currently, Bray Wyatt is locked in a feud with John Cena. He'll likely defeat the 16-time WWE Champion at the Grandest Stage of Them All. This is probably a lot better than having to lose the title at WrestleMania, which would've happened had he never lost his title to Goldberg.
After 'Mania ends, WWE will shake up their roster. This will allow Roman Reigns to feud with fresh faces before The Fiend turns his attention towards him. A WrestleMania 37 match between Wyatt and Reigns is what we think should happen because it is the show where iconic matches like this should be booked.
However, if WWE doesn't want to wait for a year, they could book The Fiend vs Roman Reigns at SummerSlam too. Both of these pay-per-views are a great fit for this match. But do you think WWE will allow The Fiend to recapture the Universal Championship in such a short time? It'll certainly affect Reigns' character.