I've put up some posts on ways to tide Diana over until they can rework her, but I wanted to put up a discussion for the rework itself.
First thing we need to decide is her role. I see Diana as the fighter mage, so I would want to move her away from the assassin role and set her up more to skirmish. That might put her top, but I really would like to have her mid. I wanted her kit to thematically oppose leona's kit, but to be very similar in approach. It sounds like Riot wants her to be more auto attack based, but instead of going the attack speed route I think some of her abilities should cause cleave to activate automatically on her next auto. Her ultimate also doesn't feel very at home in her kit, but she does need the mobility. So this is what I came up with to revamp her a little bit without losing sight of what I feel is her intent.
Her q and w remain the same, but her ult is moved onto her e and the damage significantly reduced or removed. A new ultimate is added that adds some benefit to a team, but is primarily there to empower her. Her passive should be what primarily controls her damage output, and it should start off weak and scale more into the game (maybe through ap ratios over base damage). She is still weak the first 2-3 levels until she gets her dash, but she still wont be a true powerhouse until she gets her ultimate.
Passive: Moonsilver blade - damage might need to be tweaked, or maybe it could ignore some amount of armor and mr.
Q: Unchanged - Crescent strike that marks the target with moonlight.
W: Unchanged - Shield with orbs around her that resets if they all hit something (damage down though).
E: Basically her old ult, but with no damage or extremely reduced; her next auto will cleave (still with reset mechanic).
R (New): Moonlight shines everywhere in an aoe, enemies are slowed, all of Diana's basic attacks will cleave for the duration, and her movement speed is increased in the area.
My concerns for this kit are the lack of cc outside of her ultimate, but I think it sort of fits her character. If she needs it I think her e could slow, and maybe there could be a stun when the orbs on her w pop. I think this kit fits her very well after some tweaking.
The other idea I had for her kit are to add more interactions for moonlight, but I haven't gotten a good build for it quite yet so I might add that later. The crazy route to take with her is to add some sort of time mechanic for her, and have her get more powerful during the night. No idea how that would work though, but that could potentially be a very cool thing to play around her.
Thank you for reading! Please give me your feedback on this, and any ideas that any of you have ^.^