Well, stale characters aren't over and WWE gets to look at more sophisticated numbers than we do, so I'm guessing Cena isn't as stale as you think.
Why make it "have to be a match"? Because straight up wrestling sells right? Yea, that's a load of shit. The surprise factor works and the fact that everyone has won means that anytime the champion is hurt, people get on their feet, when the music hits, they get excited. Why the fuck would you want to take away that? To "build credibility"?
It may not be a surprise that it happens the way it does but it is a surprise when it happens. Sure a lot of people predicted ADR, but a lot of people predict the MITB to be cashed in a most big matches. So it's not really predictable if people are consistently wrong.
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I have a few things to say about this:-
You say about the surprise element. But now you expect anytime the champion is knocked out or barely standing, and then the MITB winner stands on top of the stage, holding a briefcase, that he plans to cash in, and because they have won the title every time this scenarion has happened, you know the result going in.
Maybe it is about time to change things up a little and be a little creative with it.
Instead of having a heel appear at the top of the staging, why don't they come out of the crowd, or sneak-attack the champion. I think a cool way to do it is by the heel being under the ring during the match, and when the face celebrates winning or retaining the title, the heel attacks him from behind, and surprises him with his finisher. When the ref tells him to leave, he pulls out the MITB briefcase from under the ring. Maybe he could hit the face across the head with the briefcase, and because he has not officially handed the case to the referee, the match hasn't officially started yet. So, he could knock out the champ with the briefcase, not get DQ'd, hand it to the ref (who has no choice but to accept), and when the bell rings, he immediately pins the knocked-out champion for the title. This would work especially with a heel, and give them big-time heel heat for such a cowardly attack (someone like a heel Chris Jericho would be perfect for this). This would be more of surprise, as no-one sees the MITB winner come out until it is too late.
Another idea is to have someone cash in MITB, and then not win it that night, but later on. For example, a face could cash in, and go to pin a heel who has had a grueling match. Things seem backwards so far, but bear with me. The MITB winner goes to cash in the briefcase and the match starts. He goes to pick up the fallen champion in order to finish him off for the win, and gets low-blowed. The ref calls for the disqualification, and because the belt can't change on a DQ, he wins the match but not the title. However, the MITB winner could force the champion to give him a rematch, or be given one automatically for winning, but by DQ, and the MITB winner then bests the champ in a proper match at the next PPV, (mostly likely headlining "Summerslam") and then have them battle for the next couple of months. This would work especially with a heel cashing in initially, but the DQ turns the MITB winner face, as he pursues the devious champion, who cheated to retain the title.
I may think of other ideas too, but these ideas would not change the initial concept, but give some fresh ways it can be done.
This is not the Wrestlemania MITB, this is not a guaranteed shot at any title. Honestly, people, were you not paying attention when the MITB PPV was first developed? The Raw MITB match winner gets a guaranteed shot at the WWE TITLE any time. The Smackdown MITB winner gets a guaranteed shot at the WORLD HEAVYWEIGHT TITLE any time. They are specific to what brand/title you can challenge for!
As far as Cena burying MITB, I don't believe he did that for a second. Faces have always been pissed at heels for cashing in an opportunistic fashion, there is nothing new here. Cena is just being Cena, after tearing the roof down with CM Punk twice in a match to make that title mean something, he is pissed that someone else took a cheap way to steal the belt. That's it. MITB is exactly what it was before Cena spoke.
Hell, if Cena buried anyone or anything, he buried Rey Mysterio... Cena told ADR that sooner or later he'd have to defend the title and it wouldn't be so easy... He said this moments after ADR DEFENDED THE TITLE SUCCESSFULLY WITH A CLEAN WIN OVER MYSTERIO. So what the hell did Cena mean by that? That Rey is nothing?
Now, obviously I don't think that's what Cena actually meant to imply, he was just speaking off the cuff with a poor choice of words. But saying Cena BURIED Rey would be every bit as legitimate if not more so than saying he buried MITB.
All we have here is a logical character driven reason for Cena to be back in the title hunt and going after ADR, and a logical, character driven reason for Punk to for the moment not be focused on the belt but still be part of a huge, compelling, main event storyline. That's what we have, WWE using the characters there stars have to create compelling angles and feuds. Well done, WWE.
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You're right that now it is that the Raw MITB winner can only challenge for the WWE Title, and the Smackdown one the World Title, except for one factor you forgot-
THE DRAFT LOTTERY!
Now, this year the Draft Lottery happened early in the year, between MITB PPVs. Also, on at least two occassions, the brand champion has been drafted to the other brand, and taken his title with him. So, if Bryan waited until the Draft, he could cash in on the other brand's champion, if they happen to switch over to Smackdown.
Also, what if Bryan switches to Raw in the Draft, next year, but hasn't cashed in yet, can he then cash-in on Raw's champion instead? Why not, they are on the same brand now?
It would be really cool to end the Draft Lottery episode of "Raw" one year, with the MITB winner cashing in on the other brand's champion the minute he is drafted to his brand.
Also, a question, or two! Since the Draft Lottery is on "Raw", can Bryan cash in the "Smackdown" champion that night? I mean, if they are both on SD, yet it is a combined show, with both brands present, can the SD MITB winner still cash in the SD champion on "Raw", provided neither switch in the Draft? They have two or three other 3-hour combined brand shows a year, can a SD superstar cash in MITB on his brand's champion that night, and if not, why not?
Even cooler would be- The WWE Champion is drafted to "Smackdown". Daniel Bryan, as "Smackdown" champion, immediately challenges, and wins the title. The next Draft pick happens to be Daniel Bryan to "Raw", and he takes the title with him, leaving the ex-WWE Champion on "Smackdown", without his title.
Thirdly, consider that a couple of times, after the Draft, that BOTH titles have been on the same brand for a week or two, and usually one is switched to the other brand, via title change, at the next PPV (funny how that usually happens, LOL!). Now, suppose both titles are on SD after the Draft, can Daniel Bryan cash-in on either title? I mean, both are on "Smackdown", and it hasn't been determined which title will change, so why can't he have his pick of the title during these circumstances?