...Wouldn't it be funny if he lost via someone cashing in on him?

There's a possibility I never considered before until recently... Probably because the last person that a MITB winner would think about cashing in on was BROCK LESNAR. If Brock were to really challenge for the WWE Title and win it, I'd love it for Paul Heyman to actually dare the MITB winner for the WWE Title to try and cash in on Brock, knowing he won't have the balls to. But I think I have a plausible scenario where it could happen, and it would happen in the middle of the Punk/Brock feud in the fall and would be a way to turn the intensity level of that feud up to the max.

Firstly, Brock winning the WWE Title.

My original idea for Lesnar becoming champ again was to defeat John Cena at Summerslam. Cena would hold the title till then, and Cena would get into it with Paul Heyman (maybe Heyman becomes GM after ER to give him a reason for being on the show without either Brock or Punk there) and that would lure Brock out to punish Cena by taking the title from him, which he does. But since the severity of Cena's injury and whether it'll even keep him out at all is still unknown, maybe there'll be another baby face in his position holding the title. Either way, imagine Lesnar wins it at Summerslam from someone.

He goes on to successfully defend it at Night Of Champions (doesn't matter against who.) Shortly before or after this PPV, Punk returns (to a huge pop, though he doesn't play up to the crowd, but he doesn't insult them either) and says that after a long rest at home getting his head together, he's ready to return to the promised land of being the WWE Champion. This sends out Heyman to tell him that challenging for the belt right now isn't the best decision because Brock is champion. He tries to talk him into winning the World Heavyweight Championship (and having a 434 day title reign or longer with that belt) and that way, both he and Brock will rule the show as the two world champions. Punk says no, he wants the WWE Title. This sends out Brock, which begins a confrontation between the two which Heyman manages to cool down. It ends with Punk asking Heyman whether he's getting a title shot or not and he wants his answer next week. (Imagine Heyman is GM, as mentioned above.)

The next week, Punk asks Heyman what his decision is but before he can answer, Lesnar attacks Punk and puts him in the Kimura while Heyman screams for him to stop, and Lesnar lets go only after the damage is done and Punk's arm appears to be broken. Heyman shows remorse because he still cares about Punk and because we've seen in the past that he realizes how dangerous Brock is and tries to stop him from doing too much damage to his opponents (breaking HBK's arm, telling Vince to leave the ring, telling him it was enough after he attacked 3MB.)

Anyway, skip ahead a few weeks. Another successful title defense for Brock at the Hell In A Cell PPV, which is the next in the line up. And the next night is when the cash in happens (most appropriate here because Lesnar would be weakened from the night before in the cell match.) Heyman brings out Lesnar and says that last night in the cell is why Lesnar is the dominant beast that he is, and then out of nowhere, Punk reappears and sneak attacks Brock. We haven't seen Punk since the attack a few weeks before so this is supposed to be surprising and stuff. Punk has a steel chair and goes to town on Lesnar while Heyman just stands there and screams for Punk to stop to no avail. He whacks at his knees first because that'll help bring down a big man that much faster. When Lesnar is down on his knees, he runs the ropes and slams it into his throat to put him on his back. He then slams the chair into his back quite a few times to make sure he stays down. Then the MITB winner for the WWE Title runs down and cashes in on Brock to a big ovation. I have no idea who this person could/would be but just imagine someone.

After this, Punk just stands over Lesnar, having cost him the title to a quick cash in victory as revenge for what he did to him. Heyman screams at him and Punk finally breaks off his relationship with Heyman for good by giving him the roundhouse kick to the head. This intensifies the feud between the two that will lead to a match at Survivor Series (and another one at TLC surely) and also is a cool way to get the title off of Brock (who will be angry as hell as a result and want to take it out on Punk) and create a huge Raw moment in the process. I know there'll be issues with doing it that way but it would be unexpected and that's why I think it would be a great way to go.