#2. Stay on RAW: Challenge Rollins for the Universal title
The whole idea of turning Reigns heel might sound unlikely, due to the fact that he is finally being cheered as a face. The only way Reigns vs. Rollins could work is with the Beast Slayer turning heel.
Roman Reigns had to relinquish the WWE Universal Title due to his returning leukemia, back in October 2018. Rollins now has the Universal Title that Reigns never lost in the first place. So, he could claim that he deserves a shot at it again, which is perfectly sellable.
However, as a result of that, Rollins could turn heel. He could attack Reigns viciously and kickstart another run as a heel champion. Some of Rollins' best work as a WWE Superstar was as the bad guy. Rollins has a natural heel's flair (his maniacal laugh, anyone?) and would do a wonderful job at it.
However, the fans might not like it if Rollins turns, so they might boo Roman anyway. As a result, the perfect feud for him would be against...