Diana and Utility: Making her kit more like changing nature of the moon. @Vesh and @IronStylus
I saw a number of ideas on tweaking numbers regarding Diana, 
but I feel we need to change one important thing about her without the numbers. Her Q. Her Q is a crescent that always goes in the same direction, and I feel we missed a great chance to play with the idea of the waxing and waning powers of the moon and the fact that the stacks of moonlight placed by it only work with the ult. What if Diana's Q would switch which way it made it's crescent every time it hit a champion, or even if the first time it hits, it gets a stack for reduced cooldown and mana cost as well as damage allowing her to become more mobile in team fights and using her ult more often. For more on her Q and possible changes to it, check the third section in this post. For more on Moonlight changes, check the fourth section in this post.
The reason I suggest this is that Diana doesn't seem too unique to make her something that People would want to play. Yes, she has a skill shot with a unique path, but velkoz's 
Q and orianna's 
ball feel more impactful as a skillshot with us choosing a direction and making the change itself. I feel that making it more than a static skill shot could reflect more upon making Diana unique and making harder to counter her in lane by positioning to avoid the Q and prevent last hits forcing her to pick one or the other. Her shield already feels satisfying to position yourself to get all the orbs to burst at once dealing AoE damage and shielding her, and it works well with her ult already so I feel that doesn't need changing but many champs have a shield already. And her E, AoE pull towards the center and slow seems the least satisfying to use. Her ultimate is just a gap closer as well just like Fiora's 
Lunge or Jax's 
Leap or even Akali's 
ultimate. Her R might need the most attention and I'll discuss a bit about it with the discussion on Moonlight changes.
A redesigned Q that would stack and lose stacks at the full cooldown of the ability to allow it to be cast like normal but with an option to cast it about half way through from a different direction for reduced effect could make her more unique and if it builds up enough stacks one could cash them in with her ult for a full moon lunar explosion. This could change her play style from all in assassin to more of a mage with a high burst to a clean up mage, stacking Q stacks from different angles before getting one with the power of the full moon for AoE damage while diving in. Essentially making her Q have more than just one path could make her more fun to play and harder to predict, even if it's just a simple change of ever cast of it makes it swap between a counter clockwise arc and a clock wise arc could be refreshing and making it unique even without a weird cool down mechanic.
Moonlight is a weird buff, it has no use until Diana hits level 6 and even then it only is applied by one ability and is only consumed by one other. What if however Diana's third auto attack in combat would apply moonlight as well and if moonlight interacted with every single one of her abilities, modifying them slightly? This could greatly change her play style and impact and would open up more roads for how Diana could be played. If moonlight stacks could be used for other effects it might make it worth while for Diana to think twice before going Q->R->R. What if Moonlight stacks were consumed only on a target hit by Diana's W or E (powering up their effects or giving new ones only for that one interaction) and still have all them removed from everyone when Diana uses her R? This means that Diana might be able use moonlight to get an added effect on that assassin that will knock her out before diving that ADC with her ult twice, or maybe go into the middle of a team before using Q and then shielding up making her tankier or even using a Q on that bot lane support who got a bit too close to the wall and getting her E off could get an additional effect and helping bot lane get the kill without having to use her ult to jump on them for the kill.
So my suggestion for interactions would be for Diana's W to consume moonlight and make her receive reduced damage against the champion for a short duration giving her an option should she want to be tanky, have her E consume moonlight to have a stronger pull towards Diana and add on a stun or stronger slow giving her more utility, and having her ult remove moonlight from all targets to refresh, giving it more use than just "It lets me have a huge gap closer and I can use it twice."
TL;DR: Passive applies moonlight on third strike to help give more chances to bring the new utility to gameplay, Q switches directions of the arc every time it is cast or have a new mechanic that allows that to happen, W consumes moonlight against targets hit to give Diana reduced damage against that target, E consumes moonlight against targets hit to pull them to her more and stun or slow them to better break up a team, R still consumes all moonlight if hitting a target afflicted by moonlight to reset. SPECIAL/OPTIONAL: Moonlight can only be applied to the same target once (even if consumed) in the same couple of seconds that is tied to the cooldown of her Q.
