Braun Strowman will definitely happen and I for one can't wait for that encounter. Outside of that it's a bit hard to say, with Brock holding the Championship for a year, or at least being a Champion going into Mania. I'd have little issue with Braun beating Brock and Brock winning it back later. It's 50/50 booking, though I'd imagine a third encounter would end the feud in this scenario giving Brock the win but there really aren't a lot of credible wrestlers at the moment to be legitimate threats to Lesnar and having a prolong feud with Strowman allows them to attempt to change that. Outside of the most obvious answer, I also echo most of the answers said in this thread before. Quite frankly I'll be surprised if we don't see Demon Finn vs. Lesnar at some point. Also, glad I'm not the only one who wants to see Lesnar/Sheamus, I've wanted that match before Lesnar even returned I think.
However there are two more ideas I'd like to say that I'm mentioning more because I think it'd be awesome rather than I think it'll actually happen (though I don't suspect either are impossible).
First idea, is a fatal four way with Lesnar defending against all members of The Shield. There's no real reason to limit Lesnar to solely single matches, and most people point to the Rumble 2015 triple threat as Lesnar's best match since returning (I haven't seen it yet) so I don't think there's any issue here. This would ideally happen either at Survivor Series or Royal Rumble, since it'd see Roman getting as close to the Championship before Mania. I'd imagine this would happen at Survivor Series, and Reigns will win the Rumble (ugh) and then the Mania match, tells a pretty simple but effective story. The reuniting of The Shield in a match against Lesnar also lends the match an easy story to tell. The nigh unbeatable team reunites to tackle the nigh unbeatable champion, eventually egos prevail and the three teaming up start clashing with each-other. Brock Lesnar eventually gets the pin on either Dean or Seth (probably Dean because WWE), Roman can stay protected and we get a pretty awesome match. Heyman can even use this match to increase the tension at Mania, highlighting that Lesnar essentially defeated The Shield, what hope does Roman have to beat him? Creating an underdog story that fans will probably reject but on paper it's sound. The WrestleMania 31 & 32 matches can also tie into the build of the Fatal Four Way easily and even the Mania rematch between Lesnar/Reigns. I have convinced myself this is the best route for WWE to go but maybe they'll surprise me.
Second is Kurt Angle. I'm about 83% sure that Kurt Angle will wrestle in WWE again, not as a full-time competitor but on a very limited basis a la Triple H. I also know that Kurt Angle has stated Lesnar is one of the guys he's most wanted to wrestle, and with the two storied history together, it makes Kurt Angle an instantly credible threat to Lesnar's reign (even though we know he wouldn't win). This would be a SummerSlam or Royal Rumble match, since it'd have to be a big match. Continuing on with my previously built idea, it would take place at the Rumble. The story can be something of Angle echoing the thoughts of us. That his show Raw is being hindered by a Champion that leaves for months at a time, and that at this point it seems nobody on the Raw roster is capable of doing it. Or maybe Brock is starting to get really destructive on Raw after destroying the competition. Maybe he signals a retirement with the Championship since nobody can stop him. Either way, for the betterment of his brand, Kurt Angle steps up to bat, claiming his impressive record against Lesnar. You could even use this match as a way to segue into an Angle/HHH match at Mania, though I don't really want to see that and if HHH does wrestle at Mania, it should probably be to complete the hat trick by jobbing to Dean Ambrose.
Fantasy booking's bit me on the ass before but I think both are plausible and would be awesome so here's to hoping again.
