I say Roman Reigns vs. Brock Lesnar at SummerSlam makes sense for the fact that WWE's logic seems to be that a slow build to WrestleMania 34 is going to do somebody some sort of benefit in this scenario. I simply don't know what that benefit is.
If the benefit is Roman Reigns gets his fourth straight WrestleMania main event, and third straight win, then that's it. But a slow build is NOT going to change people's opinion of Reigns heading into the event. If anything, they're going to be cheering for Lesnar again, which is ironic as others have pointed out, because Lesnar isn't the sort of guy those crowds cheer for.
Honestly, with the way things are going now, I think Braun Strowman beating Lesnar for the title, and having Reigns either beating Strowman one-on-one, or both of them in a triple threat match, is much better. Strowman gets a big rub (which he will eventually deserve, if he doesn't already), and Reigns gets the title that WWE desperately wants him to have. Reigns doesn't need to beat Lesnar, end of story really.
I'm all for Reigns and Lesnar at SummerSlam if it means we get a world title back on Raw sooner rather than later. I'm starting to get real damn bored of Lesnar and the way that he is booked as a champion, although I understand it's all business. The ideal situation would be SummerSlam, but regardless of that, Reigns will most likely be in the main event at WrestleMania anyway.