This could be amazing.
First off, Tanahashi is as guilty as Lesnar for spamming moves, particularly in main event matches. I don't think I've seen a singles match from him that doesn't involve at least two or three High Fly Flows. So, to hold move spamming against Lesnar when his opponent has been doing it for far longer is laughable.
Second, Lesnar was a dud champion in Japan, we know, but he was still largely unbeatable against local stars and gaijin. By no means does that makes him unstoppable, but if I'm not mistaken, Kurt Angle was the only guy to pick up the 'w' versus Brock who was booked as a killer in Japan.
We are also in a location where Lesnar won his second world's championship so there is that working in dear old Brock's favour.
On the other hand, Hiroshi Tanahashi is the Japanese John Cena, effectively, and does the 'never give up'/'fighting spirit' shtick, so the likelihood is he'd go down swinging which is how Cena managed to beat Lesnar in 2012, he swung, knocked Brock down and then out. So there is a chance for Tanahashi.
Tanahashi's in-ring work is much more similar to say an Eddie Guerrero - fast, looks to dictate the pace, can work mat wrestling in but also uses lucha influences. We all know how Brock's sole meeting with Latino Heat went. Really its about whether Tanahashi can execute a game plan like a Guerrero or an Angle or whether he caves into the fool-hardy die on the sword type John Cena methodology.
So lets simulate this one right here:
Brock's going to try and out-maneuver Tanahashi at first, looking to keep things slow and steady, probably being canny enough to try and navigate around Tanahashi's style and take the incentive to set the pace of the bout himself; I.E. Suplex City. Given that Tanahashi has a dodgy neck, that probably hurts like a mother.
The first foray of suplexes would knock the wind out of Tanahashi's sails, but he'd get back into it with a few clever moves, say a dragon screw, and try and wear Brock down with a submission or two, most likely his cloverleaf. Brock being Brock though, he'd power out of that and try again for the suplexes with limited success.
By this point Hiroshi's figured out what 'Plan A' is for Lesnar (the much lauded "Brock Smash"). He digs deep and channels that ridiculous fighting spirit and uses it to try and instigate a slug-out. Brock doesn't buy it and tries to suplex him again, but Tanahashi manages to use the weakened legs of Lesnar to his advantage, dropping the Beast to one knee before taking the advantage with a few suplexes of his own.
He goes up top for the High Fly Flow, but Brock's up, so Tanahashi hits the crossbody variation. He begins to feel it, but still Brock rises, so Tanahashi goes to the opposite corner and hits it again, this time felling the beast. One more time he goes up top and spams that move to deliver the finishing blow. He lands, but Brock rises with Tanahashi in his arms, tosses him up top and executes him with the F5. And that's how this one ends.