As much as it is a travesty that a mage has once again been regulated to support, Zyra isn't anywhere near other champs that need to be reworked.
Her theme is fine. Lore could be updated, but that has nothing to do with gameplay. And a evil plant that takes over an area ("THIS LAND IS MINE!" -
) via spreading her babies about and punishes all who encroach on it what you would expect from an sentient, carnivorus plant(also Poison Ivy anyone?
http://grabilla.com/07a04-7ca5ad8c-3c4f-4101-85b1-d8654cf8c05e.html
http://grabilla.com/07a04-5e75dbd6-919f-4647-83bf-3d799eb3d62a.html
Her kit mechanics are fine. Her seed spawning passive could use a tweak(like having that random seed spawning an entire screen away fixed would be nice), but it helps her set up her area of control. And everything else(but the shape of q) is exactly the same as when she was viable mid.
So what changed? Her damage and scaling. Buff the ap scalings on her skills(plants especially), and trim down the base of her plants and maybe her e, and she would be heavily incentivized to go mid.