Black Cleaver's pretty much under control. It's still good, and worth buying on champions but isn't the stacked auto-buy that it was before. It feels like burst is in the right place now, it's still a valid tactic to burst targets down, but it isn't the instant explosion it was under unpatched cleaver meta.
I'd be cool with a CDR aura test. It would help mages early game, but it would help pretty much everyone evenly, including the currently strong champions who also benefit from it significantly (especially Kassadin & Jayce.....)
Blackfire Torch might still be a bit overtuned, especially on tanky APs tanky-types like Amumu & Rumble. There seems to be a fairly large damage differential there is between having it and not having it, due to the proc.
On the topic of AP item diversity, ways to do this might be to take a look at introducing more alternative build paths and scaling mechanisms for AP champions, sort of like how Ryze does. His itemization might also be worth a look too, I think it's still pretty stagnant even after he was rebalanced towards liking AP a bit more.