my problem with ap bruisers is that they're a weird ass class that needs to be actually defined. the ones you listed are all over the place, swain is like a weird dot drain tank thingie (that builds a lot of damage mind you), grago is now a utility/damage/tank support, diana's an assassin, sorta, (like drawkcab said, abyss+zhonyas an ap bruiser does not make) and new nidalee isn't even out.
Yeah, quite true. Even if Riot came out and properly defined the class you'd soon find out tons of champs lie on some sort of boundary with another class.
I mean, I'm harping on AP bruiser itemization needing help only because if this is truly an issue Riot were to ever tackle (I'm not sure they even think this is an issue, to be honest) they'd probably want to go after it holistically, which means items as opposed to re-evaluating lots of champions.
Personally I'd rather they use damage scaling from defensive stats more liberally, but there's a few examples of that biting them in the ass so I get why they're hesitant. Then again I don't see an easy way to deal with the disparity between AD and AP bruisers/tanks. I suppose that if AP tank itemization were improved, then you could make make the case that AP bruisers trade some damage in exchange for more utility (which is generally true), but the balance between AD bruiser (damage) and AP bruiser (utility) is greatly tipped to the former.