Well, you can go Leona top, I have even seen some ambitious people take her into the jungle. But in your OP, you asked for the pick the would fit better in the current meta. Taking a high utility mage to the mid lane, like Zyra, fits well into a lot of team as long as you can play against assassins.
Taking a low damage melee tank top can have issues in the current meta. If Leona goes against a champ that can kite well, or out duel her early, she will not have the tools to make a good comeback against most top laners mid and late game. That is not to say she does not scale well, but it is at an opportunity cost. She can work really well on a low gold budget, but is not a hyper tank if she does gain a lot of gold.
Playing a hyper-tank like Maokai will make better use of the gold from top lane, as his ability to peel is very tied to his gold levels, as is his damage. And Ryze or other ranged top laners will bring more damage than a Leona could. On the other hand, like I said, Leona woks a lot off her bases stats and all the extra gold from top lane does not really help her in the same way.
But in the end, it is a game, as long as no one care if you go a non-meta tank top go for it. They are both really fun champs and i don't think either one will be a bad call.