If it was up to me, it'd already be at it's end. No more matches, their feud ended at the Greatest Royal Rumble. However, I don't agree with the "pretend it never happened mentality". I mean, if the choices are "pretend it never happened" or "Roman vs. Brock AGAIN at SummerSlam" then yeah, I'll take the former any day of the week. But if I was in charge, I wouldn't simply discard that story-thread, I generally don't like when series do that.
At Extreme Rules, Bobby Lashley would become the number one contender to the Universal Championship in a match that at least would also feature Roman Reigns. I wouldn't have Roman take the fall though but he'll definitely fall short of the victory to Bobby Lashley. Then at SummerSlam, Bobby Lashley would defeat Brock Lesnar, clean, with the Spear. I don't know if WWE's allowed him to use the Spear yet, I've only seen him use the Stalling Suplex but he's definitely bringing it back for this match. And he wins with one Spear too. He can hit his Stalling Suplex finisher on Lesnar first, but he wins with one Spear. He frees us from Lesnar's tyranny. He steals Roman's narrative, and he does it with Roman's finisher.
This leads us to Roman vs. Lashley at the next PPV which I thought was No Mercy but I looked it up and nope it's Hell in a Cell, but I can work with that too. The feud here is based around Roman's "jealousy" of Lashley, his bitterness of being denied his moment against Brock. WWE doesn't have to go full heel turn, but this will at least cause development in Roman's character that will hopefully garner more interest. At Hell in a Cell, Lashley defends the Universal Championship against Roman Reigns inside Hell in a Cell. The Cell is locked, the bell rings, and then BRAUUNNNN blasts out and Strowman makes his way out. He shoves the briefcase to the ref, screams "I'm Cashing IN!" then rips the door off the cell, and we got ourselves a Triple Threat Hell in a Cell match between Lashley, Roman, and Strowman, which sees Strowman winning by pinning Reigns.
This accomplishes a three things. I. Strowman looks like an absolute monster and gets a new, never by done before, cash-in moment. II. Roman taking the fall gets him out of the Championship running, and will further develop his character towards a heel turn. III. It prevents us from seeing Lashley vs. Reigns, which is definitely a match that should be saved for a big event. From here, Strowman vs. Lashley feud for the next few months. I actually think you could have them feud til December's Clash of Champions. At TLC, assuming they don't have to have one of the TLC matches, I'd opt for some kind of draw finish. I really liked the Spear off the stage and crazy pyro and set falling that they did for Batista vs. Taker at Backlash 07, something like that. At Survivor Series Strowman will get his match against WWE Champion AJ Styles, and then he'll get a decisive victory over Lashley at Clash of Champions.
As for Roman, he'll be failing in pursuits to regain top glory. Maybe at Clash of Champions he attempts to secure the Intercontinental Championship again as sort of a next best thing deal but comes up short against whoever the champ currently is. At Survivor Series he'd probably be on Team Raw and be one of the last eliminated but he would be eliminated. At TLC, I don't know. Maybe if Ambrose is back by October they could do something together.
At the Royal Rumble, Strowman will retain against someone, it doesn't really matter, let's say Seth Rollins. The final two at the Royal Rumble are Bobby Lashley and Roman Reigns (because Reigns being in the bottom two never gets old). Bobby Lashley stole Roman's narrative. Bobby Lashley has been getting Universal Championship matches all the time while Roman's been stuck in the mid-card trying to regain his spot. Reigns is desperate to prove he can beat Lashley, everything's been riding on this moment and Lashley eliminates Reigns. Once again, Lashley stole Roman's moment and this time, Roman snaps. Lashley celebrates for awhile then Spear! Then Roman tosses Lashley over the top rope, and hell, maybe he celebrates too.
This new found "Fuck You Bobby Lashley" mentality allows him to start picking up enough speed that he becomes the number one contender for the Universal Championship against Braun Strowman (note: WWE has not confirmed Elimination Chamber according to Wikipedia, but come on, they'll have it [also SDL gets the men's Elimination Chamber to crown a N1C]). As you can figured, Roman ends up winning the Universal Championship here, but he does it in heel fashion. How so? With a Superman Punch to the DICK! That's right, the Superman Punch is supermanly powerful, and the dick is everyman's weakspot so Superman Punch to the DICK! Probably followed by his super rebound running Spear because this is still Braun Strowman we're talking about. And even though I just really like typing "Superman Punch to the DICK!" it should be noted that it's important and from here on out, I don't think Roman should be using the regular Superman Punch.
Anyway, at WrestleMania we got face Lashley vs. heel Reigns for Reigns Universal Championship. The winner, I don't think it entirely matters but what does matter is that Reigns will turn back face here. The ultimate goal with the Lesnar/Reigns story is to get Reigns cheer, so he should close the show as a good guy and you can accomplish that here. This is why I don't want Roman using the Superman Punch, only the Superman Punch to the DICK! because that can be used as a signal. Set-up a moment where the ref is down and Roman signals for a Superman Punch to the DICK! after a long-fought, evenly contested battle (though Roman had to cheat here and there early on to get a lead, the middle of the match he plays it clean) but with the time it takes Lashley to get up, Roman starts to reflect. He stops and waits for Lashley to reach his feet, then signals for it again and hits the Superman Punch. If the not dick is too subtle for a face turn for fans (it's not), WWE can simply opt for him to attempt to hit Lashley with the Championship but stop, throw it out the ring, and hit with the Superman Punch. Hopefully if the story is told properly and the fans get behind Roman, then him winning here will finally give WWE there moment, with Lashley shaking his hand at the end. Conversely, Lashley could retain, and Roman could shake his hands in respect to hopefully a cheered result.
So what does this booking I came up on the fly hopefully accomplish?
I. It avoids us having to sit through another Lesnar/Reigns match that is sure to be booed again, like seriously there plan of attack here isn't working, yet still meaningful incorporates their feud into Roman's character direction and even into Mania 35's main event, continuing the WrestleMania Narrative as I call it.
II. It gives Braun Strowman a lengthy Championship reign that he's more than earned. Certainly not as long as Lesnar's but that's another benefit, there's a few quicker reigns here to help add variety to the Universal Championship, keeping it fresh after being stagnant & mostly non-existent for a long time.
III. It firmly establish Lashley as a top guy, which I suppose doesn't need to be but definitely giving him the Lesnar win is a great rub for anyone.
IV. Roman's biggest criticisms are against how hard WWE pushes him and his character. This feud gives Reigns a lengthy extent of losing more frequently, at least losing all major matches (he'll still win now & then) while also allowing his character to undergo character development, eventually resulting in a natural heel turn. The goal here is to get him cheered as a face at the end of the day, the casual fans who do cheer him, will cheer him again when he turns. If you want him to be cheer by the vocal majority, you need to work on appeasing the hardcore fans.
V. On that note, I realize having Roman, even a heel Roman, beating Strowman might not appease the Hardcore fans but I think Roman's coronation before WrestleMania is a positive. Roman's constantly in the position of challenger at Mania, against Brock, against Trips, against Taker (albeit not for a title), against Brock. The fact that we know it's so he can get coronated on the grandest stage of them all so he can get his Boyhood Dream Shawn Michaels moments makes it feel manufactured and absolutely makes the fans more bitter to the idea. If he's Champion going in and the result isn't obvious, then they'll be more receptive to the match. Even though Lesnar won at 34, the fans turned on it before it began because of how obvious the result seemed. It doesn't matter that the unpredictable happened because WWE already facilitated an environment where that match was destined to fail.
Anyway, this is the idea that popped into my head after thinking about this question for like five minutes tops, it's not necessarily how I'd want to book WWE mind you, Roman would not be choice for top guy but this is how I think they should do it if Roman is the end goal.