Can you imagine? The absolute audacity of those madmen if this were to happen.
Right now, what's preventing the possibility of Roman Reigns from winning his second Royal Rumble is the fact that he's currently Universal Champion. He was forced to defend his world title in the Rumble match in 2016, so if WWE embody the definition of insanity, then they very well might do the same thing over again.
Still, WWE are always going to contrive a way to have Reigns chase after another coronation, so you can never fully count out the idea that WWE will be cheeky enough to contrive a way for THE BIG DAWWWWWG to lose his title, and then win the Royal Rumble to earn his shot to get back the title.
If that scenario were to occur, it would be likely that Roman loses his title to Braun Strowman at TLC in some manner that doesn't involve Roman getting pinned. Keep in mind, what I've just charted out for you comes from the most cynical and defeatist part of my WWE-loving brain.
But hey... WWE have been known to be way worse than your imagination.