I feel diana pre-6 is not that hard if your play her properly. The main issue of diana right now is she cant scale very well (what I mean here is she can't actually 1v1 most of the champions, except ADC. pretty much gg against Jax ,Yi, aatrox, Tryndamere) .Compare to other assassins (like Talon Zed, Fizz,etc those three have insanely strong lv2 or 3 kit to do one shot. pretty much braindead all in, but Diana needs passive setup and hit q to do so).
I would like RIOT just give her back the role she should be (high risk high gain, once scale up, should be unstoppable, 1v1 potential mid and late game).
One thing that actually can bring her back from 45% winrate, is giving her ultimate like Akali reset / or immune time during dash (so she can penetrate veigar's E ,Ahri's charm, brand's q, etc, I think its reasonable change , you needa time it perfectly to dodge skillshot~, and btw there's no way poke champions can have such strong cc to actually counter divers, makes no sense when they can already do insane dmg long range )
As a diamond Diana, I always build Full AP, with 0 defensive item. The saddest thing for me is when I build defensive item like ROA or Zhonya, the dmg is not sufficient to be a threat, one of the reasons is mana cost, 10 lv your mana can only allow to do one and a half combo (reasonable but dmg is very insufficient, Fizz even miss Ult, just QWE can do more than Diana. feelsbadman).Second, With her ultimate, it always bring her a very dangerous spot(RN is high risk low gain).
I'm looking for her rework, i hope RIOT can do better balance work in future patch, so every champions is playable and also takes skills.