Changes to Diana's current state
She needs a change to her kit's mechanics. I've played diana enough to the point that she's either super hit and outright nukes a vulnerable target, or super miss and dies trying without any true means of escape within her kit. Her kit is interesting and fun, but when she isn't building damage and tankiness at the same time (zhonyas as core along with abyssal) it's pure feast or famine in nearly every one of her skills. She's too risky as she can blow one person up, but can be blown up just as easily in a teamfight as she has no way of escaping, although every other assassin in the game does.
Transform Diana into an AP bruiser/fighter with a means of escape. I'm still not sure if diana should even get an escape. If she doesn't then her shield numbers should be tweaked to create a better well-rounded bruiser.
I'd make Diana's Q have a higher base damage, but lower AP ratio to make her actually be able to get some poke in and trade a bit before she gets her power spike at 6. Morello has stated that if diana's crescent strike were to get a base damage buff, it would need to be slower, but i don't think that's the case. Although its range is high, it isn't that difficult to dodge as it's predictable the moment you see diana's blade flail outwards. Base damage (60/95/130/165/200)-->(70/110/150/190/230). Ap ratio reduced to (.5) from (.7) Mana cost increased (55)-->(50/60/70/80/90).
Diana's W also has a current issue, the AoE at which the orbs blow up is extremely low, and it's difficult to get the second shield proc if she's in a 1v1 duel. The only changes i'd like on her W is to have the orbs in a larger AoE and spin faster so she can trade better by knowing how much effective hp she has within the duel. Also maybe make them have a weaker shield early game, but a much stronger shield late game. After all, shields such as orianna, lulu, and lux shield for quite a lot late game once they get enough AP going, and those can hop to another player. Diana's self-shield should be in the same tier as it has a pre-requisite for it to occur. Maybe make it so that the first shield is weaker in base than the second, and make the second shield stronger than the current one, as the first shield is on instantly, while the second requires the orbs to pop for it to activate. Magic damage per orb (22/34/46/58/70)---> (20/40/60/80/100), orb damage ratio reduced (.20)-->(.15) per orb. Orb speed and AoE increased (200)-->(250), initial shield ratio (.30)--->(.15). Secondary shield (40/55/70/85/100)--->(40/80/120/160/200), secondary shield ratio the same.
Now the largest changes would be to her E and her ultimate. I would personally make her E in a slightly larger AoE (250--->300) so she has some impact in teamfights, and make it so that the further you are away from the center of the AoE, the more you get pulled in, however the slow lasts for 1-2 second(s) instead of two, and the closer the enemy is in the AoE, the less they get pulled, but the slow lasts 3-4 seconds. Someone also suggested to make the AoE larger per level, which sounds great in theory, but may become too strong as it's already a non-damaging orianna ultimate+W. Cooldown (26/24/22/20/18)--->(20/18/16/14/12) AoE increased (250)-->(300) Mana cost reduced (70)-->(50).
Now for her Ultimate i would maybe shave off 10% of the ap ratio and increase the base damage, and make it so that the second ultimate, if used on the same target, deals 60% of its original damage, as to make her less oriented of a single target nuker, and give it a purpose to gap close and proc your passive more frequently rather than use your ultimate twice for the most damage. Another change could be that Diana can use her ultimate to an allied minion or champion, however the CD would be increased by 5 seconds as to assure that she can't get out quickly and dive back in once she's safe, similar to lee sin's W change slated to be on the PBE. Also if her ultimate resets, the second ultimate costs no mana so that she can stay in a fight longer to cast more spells if needed. Base damage increased (100/160/220)--->(150/200/250), ratio reduced (.6)-->(.5), cooldown decreased from (25/20/15)---> (22/18/14).
Tell me what you think. By the way these numbers are just ball-park numbers. If any of these changes even go through i don't expect the value changes to be this drastic. Diana's kit is very well-rounded, it just needs some tweaks in order to bandage up her playstyle as a tanky-bruiser-mage-assassin, and turn her into more of a bruiser than can duel very well and survive in fights if things get dangerous. Although some of these changes may push her over the edge, i think in the current state of the game it would make her a healthy choice if a team needs a bruiser or a tank that can still disrupt the enemy team and deal good sustained damage.