What's wrong with another Treant? What you've suggested doesn't make much sense, and here's why:
Riot is trying to establish a living world of lore. Characters like they were done in the early days, where it's like "I like this. I'll make this." left the game with 40 different characters who were randomly put together in the same arena. Riot is avoiding that style of creation and attempting to make champions that flow and fit better into the established lore, and in the cases of Kindred and now Ivern, they're expanding into entities that were random one-ofs and trying to show that the world is full of creatures that operate very differently from the previous entities.
In Zyra's lore, it all but tells us that she's the last of her kind, so having more plant creatures like her wouldn't fit without considerable changes, and just adding other random plant things would just add one more piece of disjointed lore to the mess they're already trying to harmonize.