A Twitch Leona lane is optimal for Leona. I cant think of any other ADCs that are good with Leona simply because of her passive. Maybe and Ez or a graves, double burst.
However, I think the complaint made by the OP is valid. Leona is pigeon-holed as a support tank. Her early game damage is great as long as she is aggressive. Her mid-late and late game are terrible, she has no damage. There are no optimized builds for her besides building health and being a brickwall. And Riot has nerfed health and resistances, making being a brickwall harder.
As a Leona player, I think Leona needs a slight rework or buff or stat changes to open up her playstyle.
Edit: Forgot to mention that I would like if leona got something for putting sunlight on things