I'm concerned about Diana's state of play once Ekko is released, and I'm surprised all comparisons have been of Riven + Zilean when his whole kit is a better version of Diana's.
Lets take a look.
Diana: Ranged poke that also allows a CDR reset on ult dash.
Ekko: Ranged poke that slows units and damages units both on cast AND return.
Diana: Self-casting AOE damage that provides a shield
Ekko: A free casting large AOE slow that if he enters provides a shield and stuns all enemy units in the radius. Also has a passive component that increases damage to low health targets.
Diana: Self-casting AOE pull for all units (Ekko's W does this and her W in one spell!)
Ekko: Short range dash and long range point>click jump to enemy location that empowers the next AA.
Diana: Ranged dash to enemy location.
Ekko: Ability to return to position/partial health up to 20 seconds previously.
All of this makes Diana look pretty lackluster on paper, he's able to do a better job of her entire kit pre-6 and then has bonus damage in both his passives plus that ultimate! What I'm hoping is this means Rito are close to revealing their planned kit rework of Diana soon since this kit is not only identical but much stronger. Maybe the damage numbers won't be up to her scaling on Ekko, but the utility easily outweighs this.