I think they want her passive to revive her, so when she's ahead she's noticeably more strong but there's two issues.
Zyra has issues getting ahead
Her Q has an INSANE delay time for being her main damage and also having a tremendous mana cost, and also not doing that much in comparison to someone like Annie's Q, which is also her primary damage. Her E is pretty easy to dodge, but I'll admit It's easier to hit. And her ultimate has a 2 second delay before the knockup. Zyra has so much counterplay to her abilities, there's literally no reason you should be hit by her Q in lane unless she's on top of you. With a .625 second delay, It's so easy to simply walk out of Zyra's Q and not be hit at all. Just buying even a basic pair of boots makes it impossible for Zyra to hit you with her Q. Zyra needs buffs on her reliability. It's so easy to simply walk out of ALL of her abilities.
Zyra's passive isn't that effective itself
Again like I previously stated, It's insanely easy to dodge all of Zyra's abilities including her passive. Her passive has a 2 second delay, a really bad width, and also doesn't do that much damage. It does true damage, but it won't really scratch a tank. They need to make her passive realistic. When Zyra dies people should be like "Oh dang, we should run away Zyra could kill us." rather than "wait, zyra has a passive still?". I think the only reasons I've hit people with the passive is because they forgot about it, or just simply didn't care enough to dodge it.
Allowing Zyra to revive with her passive would be an awesome idea, but for it to be actually useful is a whole different story.