Who should be his opponent?
Let's start by saying that Lesnar on RAW, which means, unless WWE pulls some creative "magic", Brock Lesnar has to face a guy from RAW.
RAW only has one viable option for Brock Lesnar: Kevin Owens. I'm putting my money on Kevin Owens to becoming a big babyface in the future. I think, Kevin should face Lesnar in an equal match, only with the match ending with literally Lesnar TKOing Owens brutally. Kevin Owens's spirit and the result, will bring sympathy for Owens and plenty of cheers. And no, Lesnar shouldn't lose yet, that should come at Wrestlemania 34. But, If the match is a solid one (not a squash) and if Owens is posiotioned as an equal to Lesnar, even if Owens ends up losing, he will have gotten over.
Especially if the loss comes in a brutal way that will make the fans sympathize with Owens.
Also, Owens vs Lesnar can be billed as an acual fight, given Owens' gimmick and Owens and Heyman will really sell the match in the build up. It will be golden and it has the potential to have a big match feel, especially if Owens' stardom continues to rise from here on out.
Why not the other choices?
We have already seen Roman vs Lesnar, even though the match needs a closure, Reigns won't be the one beating Lesnar, not if the fans get behind Reigns. So Lesnar facing Reigns in a predicable match result isn't really all that great.
Joe, Balor and Nakamura won't have the momentum and popularity needed to get in the ring with Brock.
Finally, there's really no interest for Brock vs Rock or Brock vs Goldberg. WWE sells out the "New Era" and there should be matches advancing the New Era or at least the present one. Goldberg is from 12 years ago. The match will be predictable and I don't think people want Lesnar vs Goldberg. All they want from Goldberg is to get in the ring, say "You're next!", spear, jackhammer, pop and leave.
So I'm going with Owens in a story that if booked correctly can be money.