Arkham has a staggering number of different mechanics, and pretty much all of them are based on a more or less solid thematic and mechanical foundation, but a number leave something to be desired in terms of execution. Fortunately, it is (theoretically) possible that some of these mechanics might be rescued by a revision or the addition of a new rule down the road. (For instance, the King in Yellow Acts, which, although they worked alright with a small Mythos deck were, so to speak, lost in the shuffle when you played with a large number of expansions, were rescued by the new mechanic introduced in Miskatonic Horror). That in mind, which mechanic would you select to receive a similar rescue? While general mechanics are the most likely targets, realistically speaking, feel free to nominate an individual card as well. If you feel like it, you can also enumerate how you would do it, if you suddenly found yourself body-switched with one of the designers (who, as we all know, are some of the most powerful sorcerors living today). Oh, and to prevent all the responses from being Call Ancient One, I would reinforce that the mechanic is one you see potential in and dislike the execution of, not just one that you generally dislike.
Personally, I'm tempted to go with the Bureau of Investigators, simply because its execution is so stunningly lackluster. However, it ultimately cannot be anything but Corruption. The idea and mechanics are so very compelling, but it's difficult to get any decent amount of Corruption even just playing with Black Goat and the base game, and nearly impossible when you play with all expansions as I do (unless you're playing Lola and specifically gunning for Initiate, and even then, the Skill deck is pretty large). The Black Goat itself doles out plenty of Corruption, but it also doles out a number of other things (well, just one other thing, really) that makes playing with it less than ideal for anyone whose user name is four or more characters. Fortunately, it would be pretty simple to fix, and there's a decent chance FFG might take a swing at it in future. Although Corruption was introduced with Black Goat, it's not exclusively thematically linked with Shub; a number of the Miskatonic Black Goat encounters in Dunwich give you Corruption. Thus, it would be a fairly simple matter to bring out a new expansion with a Herald or mechanic that distributes Corruption.