Diana should have gotten the project skin.
She has, what, 2 skins? Leona had like 4 before her project skin.
Why does Riot like giving one champion 2 - 3 skins in a short time frame while other champions only get one every few months, if they're lucky?
The sensible thing to do would be to have a rotation, one champ doesn't get another new skin until all other champs have gotten one.
IE: Zed just got a skin, Zed doesn't get another skin until EVERY OTHER champion has gotten at least one. So on, so forth.
None of this "Let's give this champion 3 new skins within a period of 6 months, and let that champion have one skin for an entire year or more."