I am still going with Lesnar to retain. He isn't going to lose to Roman Reigns in a fatal fourway. They have been building towards the Reigns and Lesnar definitive encounter ever since Lesnar ended the Streak. Three years of build, having their first encounter end with neither winning, having both defeat the Undertaker at WrestleMania, Lesnar being the most dominant over the Undertaker of anyone with several matches against him, Reigns retiring Undertaker on the same event Lesnar won the Universal title... just to drop the belt to Reigns in a Fatal Fourway at Summerslam... I mean honestly, this would rank as one of the stupidest decisions that WWE would EVER have made in my opinion.
If Lesnar loses to Reigns, it will be in a one on one match and it will be at WrestleMania. The idea that they would fast track this feud simply because Lesnar might not sign again after WrestleMania is silly. They have him on contract through WrestleMania so he will be at the event. Then, they can do Reigns vs Cena at the next WrestleMania. Why fast track two of the most interesting match ups Reigns could have and piss away all this build by having Roman win at Summerslam?
Lesnar should retain the belt and they should continue on the path they are on. Now. although I would rather see Strowman or Joe win, I just don't think it will happen. I guess it's possible if they want to have a Universal title match more often and they can give Lesnar the belt back later to lose to Roman, but that's seems unlikely to me.
And I have no doubt Reigns will not win the Rumble. He doesn't really need to. If Lesnar is still champ, Roman will automatically get the title shot and they will justify it with the whole dominance of the Undertaker thing. This way, they can have another wrestler win the Rumble and challenge for the WWE title. This is the benefit of having the two World titles... they can try to appease two groups of fans. There have been plenty of times during the first brand split the Rumble winner didn't headline WrestleMania, wrestling lower on the card. I feel this is what they will do here.
And if I am wrong, I won't lose much sleep over it. It's not like WWE hasn't done things that have completely baffled me in the past. I just don't see them messing with this set up. There has been way too much build.