What about Samoa Joe? He can fit well with Brock's style, all it really needs to be is a stiff, intense, hoss fest and the crowd will fucking love it, think Ishii vs Shibata at Wrestle Kingdom 10. Plus Joe can look like a million bucks if booked correctly when up against Brock.
He needs star power more-so than a suitable match. Lesnar's style meshes well with any style. I do think Samoa Joe would be a good opponent, by a WrestleMania caliber match will not be against him. Lesnar's big paycheck requires a big match, and I just don't think that match would be big enough for the average fan to fork over money for. It's definitely something that should be tried, in a few years time. If Samoa Joe is booked correctly, and presented in a similar light to that of Taz or Brock Lesnar, for that matter, it would make a match between the two, a real draw. It would be a great match too, but he needs to be built up to the casual fanbase first.
As for Lesnar, I think he'll be facing someone that has the popularity but still a uniqueness of a fresh opponent. Names that come into mind are Finn Balor, Braun Strowman and Kevin Owens. I think they fit the mold well, for newer opponents for Lesnar. They all offer different takes on the whole feud, and it would be interesting, if either of them had the chance to face Lesnar.
I think the main reason I think Storwman against Lesnar would be suitable, because it's a monster heel vs. monster heel. One's a legitimate bad ass and the other is a disgruntled bulldozer. I can see the scenario play out, as Strowman wanting the toughest competition in WWE. He would've mowed through a lot, if not the entirety of RAW's mid-card roster. Rather than go for the title, he goes looking for competition. He would choose Lesnar, and it would be presented, that Lesnar is actually at the disadvantage for once. Think the overall feel for the fight would be too "80s" for the average fan to really enjoy, but I don't have a problem with a good ol' fashioned "80s", big boy fight :dawg:
As for Kevin Owens, I think they could do the Seth Rollins v. Lesnar feud that never really ended on a good note. Kevin Owens' ego grows too big for the control of Stephanie McMahon and even Triple H, so they turn to find a mercenary to bring him down to earth. I think the match would be a good way to solidify Owens' gimmick of fighting when it matters. The match would be fantastic if Owens were to get offense in, but the only real downside is that some may perceive it as Lesnar being cheapened down. Like he man-handled Cena only to be 50/50 with Owens? Again, with correct build, this match would work, but I do think it would work, despite the lack of history or similarities between the two.
Also Finn Balor seems like a good opponent, because he offers a completely different dynamic. Though Lesnar has had feuds and matches with cowards, monsters and legit fights, he hasn't had a feud or a match with a guy like Balor. Balor is a demonic babyface, who doesn't need heart and soul to win, but rather his demonic side. It's something that a Lesnar match or feud has had. Have Balor and Lesnar get pit against eachother in an exhibition style fight, and build it as Lesnar going into the eye of the storm, and challenging the demon. It will be The Demon vs. The Beast. Truly would be an amazing match, too.