#4 Most logical opponent right now
With Roman Reigns, we can never be sure when he's going to return. Tom Colohue reported that WWE had to scrap a lot of plans with Roman Reigns gone and there's no telling when he will return.
For all we know, Reigns could be off TV for months. It's not that he's sick, but he reportedly felt 'dizzy' in his last set of tapings before WrestleMania 36 and requested to be withdrawn from the match against Goldberg.
Colohue stated that there are no plans right now to bring Roman Reigns back. He said that WWE themselves aren't certain as to when he will return to the ring. However, given his pre-existing health condition, it's probably better that he stays at home rather than attending tapings.
Right now, WWE programming can do without him and with The Big Dog not around, this frees up space to have the most logical first rivalry for Braun Strowman. Simply put, there isn't anyone bigger on the roster to challenge for the Universal title.