I think we all feel this in some form or another, although certainly not everyone would agree that playing against Diana sucks. Personally I find playing against her to be quite easy, as she is only really a danger if you fail to understand when she can kill you and when she cannot. Otherwise she's quite easy to beat in lane, and even easier to gank.

Anyhow, in my opinion, there is no such thing as an AP bruiser / fighter as there are simply no items in existence for this. Take a look at potential build path for an AD bruiser in top lane. Compare that list of items with what you might build on an AP bruiser. It just doesn't exist.

As has been discussed ad nauseam in recent posts, there's no point to building anything other than AP on Diana as it as her kit is entirely built upon AP scaling and burst damage rotations. This isn't my personal opinion, it's reflected in every plat+ Diana player in every region/server on earth.

I think you're totally correct that Riot doesn't want to fix her. Her issues have been known for a very long time, and her "mini-rework" was (in my opinion) a total farce, so it seems unlikely that anything will change on her as assassins in general seem to be being marginalized, with a much larger emphasis on slow dps and/or control mages, which make for better "gameplay experience" apparently.