How to bring Diana to other mid-laner standards
Changing her ratios, cds, base stats, etc won't hold much momentum. Editing those things will simply over or under compensate her true problems. I believe the answer is simple:
Make it so that using Lunar Rush (r) doesn't remove all of her Moonlight stacks on other targets, only on the target that was dashed to.
This will give her more mechanical gameplay and make clear the skill difference between the better Diana players. She's often played mid, so she'll need to build a lot of damage to be effective. She's also melee, so we'll need some survivability which takes away from her damage. She has a dash, but it's a 1-way trip. If she doesn't land her mark and 100-0 the target, she will have effectively done nothing. But if i land a multi-target Crescent strike, then i can make some plays. I've still got the 100 mana cost to worry about, combined with my low mana pool, but i'll make due. If i could dash through a few enemies, then make it to my target, dish out some damage, maybe not killing them, and then leave the fray in a similar manner than i entered, we'd be around the same level as some other mid-laners.