Personally I love the two plant/nature themed champions that riot has given us 
But I would really like to see more
for example how come we don't have a support that would be like zyra mixed with soraka. full of heals with a snare but nothing too OP
or have a champ like 
's earthrune skin that is animal-esc but is a mass of self motivated plant life
or one that looks like red buff (my personal favorite camp on the new rift).
anyway just a thought
The problem is that every champ doesn't just need to be unique in terms of gameplay,but also thematically. You see basically every new champ has one or more archetypes that are unique to him/her(rek'sai is a tunneler with elements of a sand-worm,kalista is a vengeful spirit,azir is a sand-mage/emporer/ra-look-alike,gnar is a prehistoric guy/guy with hulk-transformation,etc.)
In Maokais case you have the treant,a living tree In Zyras case she is more like a plant-mancer(I know she is actually a plant herself) and gives off a poison-ivy-kind of feel,focusing on thorny-plants and so on.
The problem is that if they make more plant champs,then they need to be thematically(and gameplay wise) different from these 2.
I guess a flower-based champ could still be a possibility(albeit it would be a bit difficult to differentiate him/her),or some kind of druid(that one would actually be not a plant itself,but might use plants as a part of his/her kit).
Mushrooms are also a possibility(I mean the only ones in the game are teemos),I know mushrooms are technically not plants,but they are close enough.
i would also like to more Nature (in this case plant & earth elemental theme) Champions & Riot could go with a Champion that is made of that element or simply someone who can control it. like they can go with a fungus-themed nature Champion from the jungle for your plant or maybe a molten magma that can become unstoppable rock-themed earth Champion. i have a few Plant-themed Champions if you want to look at:
Gore-Bog, Thing from the Marsh http://boards.na.leagueoflegends.com/en/c/skin-champion-concepts/9AgvxKig-gore-bog-thing-from-the-marsh
Lila, Warden of the Forest http://boards.na.leagueoflegends.com/en/c/skin-champion-concepts/5LmOaw94-lila-warden-of-the-forest
Suolo, the Living Land http://boards.na.leagueoflegends.com/en/c/skin-champion-concepts/OcJRqE2V-suolo-the-living-land
Gormon, the Forest Keeper http://boards.na.leagueoflegends.com/en/c/skin-champion-concepts/dIqEjK0Z-gormon-the-forestos-keeper
Ceil, the Moonlight Blossom http://boards.na.leagueoflegends.com/en/c/skin-champion-concepts/ftj4zurW-ceil-the-moonlight-blossom
