Zenith Blade needs to be buffed.
Why? Well here is a list of things that completely hard counter Leona and make her useless(Assuming her enemies know what to look for vs Leona)
Shen Black shield. Morg ult. Dark Binding. Lux root. Lux Q Xerath stun. Taric in general. Alistar in general. Pyke in general. Nami Nautilus. Draven knockback Ezreal Vayne Kaisa Sivir Tahmn Kench Thresh Zilnean Lulu Caitlyn Lucian Jinx Annie Lissandra Zyra Zoe. (This is just the stuff in bottom lane alone).
What are the instances where Leona is actually a viable pick vs competent players?
Any combination of Brand/Sona/Soraka with Varus/Kogmaw/Jihn/Ashe.
My proposed buff- Increase the missle speed of Zenith Blade, and attach a VERY brief CC immunity upon it latching onto a target. Its currently out ranged, out speeded, and out utilitied by basically everything in the game and her entire gameplay revolves around it.
The reason why Leonas E has always been tuned to be so slow, avoidable, and counterable is because she was meant to be the burst damage melee support. Which she was. For a long time. And she was fine, she was balanced, she did her job. Well...they took ALLl her damage away. Literally ALL of it. But they didn't compensate her in anyway. So now her E is just slow as crap, and dreadfully easy to counter, for no reason. She is no longer a threat, her kit should be more usable now, since it isn't so high threat.