I don't know what they should do now really, because no matter who wins it seems far from ideal.
Reigns is painfully unprepared for this main event. I'm not a blind hater of the guy. A year ago if you had asked me if I'd rather see Reigns or Bryan main event Wrestlemania 31, I would have said Reigns. I probably would have said it even six months ago. But they did nothing to prepare Reigns for that spot. He never got over enough with the fans as a singles performer, they didn't let him work any interesting angles and he hasn't even feuded with Lesnar at all. They've barely interacted. How often is the main event of Wrestlemania not booked as part of a personal feud?
If Reigns wins the title, the fans will shit all over it. His career can go one of three ways from there. They either realise their colossal error and depush him and his career is ruined forever. Or they keep pushing him regardless and create an even more horrific version of John Cena: someone who has no in-built fanbase and was disliked almost from day one, which I have to believe will spell disaster for their ratings. Or they turn him heel.
Having him win and remain face is therefore a horrible idea, no matter which way you slice it. And given that Reigns' future headline status in WWE is far from certain right now, I'd much rather we not crown another champion who fans can't even remember 10 years from now. That stuff degrades the title and the company's history.
Turning him heel to win the championship won't be much better because he'd be turning heel on Brock Lesnar, and Lesnar cannot work as a face right now. He's not around enough, which makes him appear entitled. Which is not a face trait. He needs Paul Heyman to keep his stories going when he's off television, and nobody is going to look like a face with Paul Heyman as his manager. And finally, everything about Lesnar just says heel right now. He's ruthless, he's dangerous and he doesn't give a shit about anything. That character cannot be the hero. It doesn't work.
And again, Reigns isn't working as a face, but that doesn't mean he'll necessarily work as a heel either. Putting the title on a guy who has so far been a failed project is still a risky venture.
The alternative is that Lesnar keeps the title and that doesn't sound great either. Lesnar has been the best champion in years, I agree. He looks great, and the belt looks great. But the show sucks worse now than it has maybe ever, and I don't think those things are mutually exclusive. We've done the whole absent champion thing, and we've suffered from it and now it's time to go back to a champion who's going to show up.
Besides, Lesnar's reign has gone on long enough. It should end soon. Very soon, or it will get tiresome. It's time for him to drop the belt. Or a memorable reign risks ending in an anti-climax (though that might be unavoidable at this point, considering how they've booked his Wrestlemania match).
If I had to choose, I'd say have Lesnar win. But make him drop the title over the next month. Either at Raw after Wrestlemania or at Extreme Rules. What you do with Reigns from there, I don't know. I'd try to pull him into a feud with someone else to get him away from the title for a while. Maybe turn him "tweener", much as I loathe the word, to see if a badass face character can be salvaged or if he should turn heel.
I still think the best option WWE has for the title in 2015 is to put it on Rollins soon and have him work with all the faces on the roster. If he cashes in on Lesnar, he can have pay per view matches with Orton, Cena, Bryan, Ziggler, Ambrose, maybe Reigns, maybe Lesnar. He could hold it until Survivor Series and have a really good run. That's not possible for Reigns or Lesnar right now.