Can I ask a question on the design philosophy side of things, which may in turn help target what Zyra may need? Full disclaimer, I love Zyra, and would absolutely love to see her midlane again.
When I asked about Riot's opinion about Sion, you said you felt that he was balance number-wise, and his winrates were reasonable. On champion.gg Zyra and Sion both have two role entries. Both have very close play and winrates. Sion Top has 51.12% win and 1.73% play with Zyra sup being at 51.84% win with 1.54% played. Their off roles are 48.37%/.44% and 49.77%/.51% respectively.
So considering the performance, I think the best way to start is asking what is it about her play that makes Zyra feel weak, when her performance is in line with a champion that feels appropriate?
Personally I hate her W for the fact that there's no reason to upgrade it. If it gave her plants extra health for upgrading it, that would be really cool.
I know others don't like her passive, and would like to see that changed. The best idea I heard for a new passive was one that would help her Solo lane without really affecting her Support power too much. Make her passive spawn plants when she last hit's a minion with an ability. Melee minions spawn short ranged plants when killed and casters spawn ranged plants.