#4 Paul Heyman turns on him
This would also be a left-field move but one that makes sense, if you think about it.
It's pretty well known that Lesnar's departure to the UFC is imminent but instead of losing the WWE gig when he's gone, Heyman secures his immediate future in the company by siding with another Superstar - presumably Roman Reigns - and costs him the Universal Title.
Charlotte Flair to return and wrestle a CURRENT CHAMPION?
It would freshen up both the Lesnar character and the Reigns character, which seems to have run its course as a babyface following the defeat at WrestleMania 34.
And with Heyman in Reigns' corner, another match between Reigns and Lesnar is always a possibility when the beast returns to the WWE after taking care of his business with the UFC.