Mkay. Big post incoming.
Diddy is not a "gimmick" character. While this may have been the case in the past, none of my 'gimmicks' work any longer. Canceling a peanut gun to a banana is no longer an auto combo, people don't crumble at the sight of bananas, (except m2k), side bing onto an opponents shield anytime I am holding a banana no longer garners an automatic 15%, and ADHD's famous "camp trap" isn't really a valid base strategy any longer.
The notion that people will all of a sudden get better with bananas at this point in the meta game is an absurd one. The game is about two and a half years old, and about a year or so ago, Diddy was listed as a meta knight 'counter.' Wouldn't any sudden breakthroughs regarding banana control have come in by now? If players are not good with bananas at this point, it is simply his or her fault, and frankly this isn't the case at top level play.
Banana control is the least of my concerns at high level play. I played against Ally, Judge, and Krystedez this past weekend and banana control was never really an issue. Diddy Kong's item micromanagement capabilities are hands down better than everybody elses. Some characters, like ROB and Meta Knight come close, but frankly diddy's dash attack, air dodge, and aerials are superior regarding handling items. Some characters, like game and watch and King Dedede have an extremely difficult time picking up a banana, particularly a grounded one.
There was a jump when people began to learn the matchup-but that time seems to have passed. And the only matchup that seems to have gotten significantly worse is Meta Knight, and that's partly because MK isn't used to having to study matchups that hard ^_^. At one time I believed that Diddy's matchups would significantly worsen--but I think the time came and went and that his matchups did not get much worse. Although many low level players may not properly be able to fight Diddy and may feel overwhelmed, at top levels of play most seem to have the Diddy Kong matchup fairly well locked down.
All this being said I do agree that Diddy probably will drop a few spots on the next tier list, but I do feel this is partly due to the lack of Diddy players despite Diddy Kong's solidarity as a character. He is difficult to pick up simply because playing him is almost completely different than playing any other character in the game.
I talk about Diddy's matchups and shiz related to this discussion in this interview that clowsui and his new smash content website gave me last weekend.
http://www.youtube.com/watch?v=_Asti1zgEPE
Plz ignore how awkward I am
Although some players know the Diddy matchup than others, it is extremely rare to run into a high level player that still falls for Diddy Kong "gimmicks."
Edit: Diddy does indeed have a CP problem, but is mainly an issue against Meta Knight and possibly Wario. I think those are the only two characters where I feel helpless on their CP. (AKA brinstar, RC, and norfair if legal)
Banning of MK might make Diddy worse due to him seeing a lot more of our more difficult matchups, but by the same token many of Diddy Kong's matchups I consider to be around even anyway.