Seth Rollins
While Seth Rollins vs Dean Ambrose is the feud everyone points to when discussing the break-up of The Shield, everyone seems to forget that it was Rollins and Roman Reigns that held the tag titles together when the faction was first running rampant on RAW and SmackDown.
We're not sure what the alignment of both men would be, and to be blunt, we don't really care. As former Shield brothers, you can bet Rollins and Reigns know each other like the back of their hand, and while Ambrose aka Jon Moxley probably wouldn't be there to see the chaos unfold, you just know he'd be watching from a hotel room somewhere.
Seth is the only man, aside from Brock Lesnar, to actually be featured in a truly great WrestleMania match against Reigns. While Rollins may not have been too heavily involved in proceedings until late in the match, it wouldn't take much convincing to persuade the WWE Universe that they'd have an absolute barnburner of a showdown in a singles bout.