Leona is fine in her role as an aggressive support
For Leona to go into a solo role, especially top lane, she would need a large scale rework to most of her kit. Because while she is a tank, she is missing reliable waveclear, a passive in a solo lane, any form of sustain, and some sort of trading mechanic, because all of her skills right now are about locking down one target, not about trading with it.
So, for Leona to go top lane, you would have to completely change Leona to the point where she wouldnt be the same champ that she is now, and she probably wouldnt be able to remain as a support. So if you want to sword and shield top laner, hopefully Riot will get around to making one, but I honestly doubt it as Riot considers that to be an overdone trope