Making Diana viable/decent
Change proposal: E: Moonfall: Diana pulls all nearby enemies to her, and slows them for 2 seconds. If an enemy is affected by moonlight from Diana's Q, they are stunned for 0.75 seconds upon landing and the moonlight debuff is removed from all enemies. (For comparison, Riven's stun is 0.75 seconds)
Explanation: In my opinion, Diana is an excellent assassin. She has good counterplay, great damage, and is high-risk high-reward. Diana isn't an assassin though. She's this AP fighter assassin hybrid. And I don't think there's anything wrong with that. She's kinda like an AP hybrid of Lee and Riven.
Assassin Diana is perfect the way she is, but if she falls behind she isn't really viable. She'd be better suited building tankier items and trying to help her team. I want to make bruiser Diana viable by giving her a small amount of CC to let her engage for her team, peel for her carries, and donate kills more easily. It would also be a slight buff to assassin Diana by giving her a tad more power pre-6, and helping her escape after assassinating someone.
Diana doesn't need more damage or range or mobility in order to be viable. That would tip her into overpowered. Diana needs versatility, so that in both games she does well in and games she does poor in she's effective, not just one of the two. I think adding this slight additional CC will give bruiser/tankier Diana the viability needed to make Diana as a whole viable, and it's only an extremely minor buff to assassin Diana.