Then what about Sion's passive, how does it add to his kit after death?
Refer to the first point, it fits with his overall theme far better than Zyra becoming a suicide sniper. But if you want to also delve into his gameplay, it's still far more fitting than Zyra's because of his tendency to be deep into the enemy team, and for him, taking damage and even dying for his team is part of his function. Obviously you'd want to live, but tis life of a tank, especially with all the Last Whisper's running around. When he is dead, he is in prime position to go after whoever he wants. He actively puts himself in harms way as per his function, and he is given a chance to actually inflict real damage when the enemy thinks they finally took him down. Not to mention he is able to tank a few more times because of the Passive.
Ironically Sion's Passive that ensures his death somehow lets him tank longer and better than Zac's Passive that the enemy can kill without realizing due to their abundance of AoE abilities.
As for Zyra's being on her (and Kog'maw too), it's literally nothing but a lazy cop-out because they wanna be done with their work and just release the champion as they are.
What about that Passive makes it so that it works all that well on Zyra? How does it in anyway syndergize with her kit at all?
In return, I'd like to ask if you have any suggestions on what you feel her passive should be then? Even if its just a general idea.
Consider what sets Zyra apart from other champions. For her, it's fully autonomous but temporary pets. She has probably one of the most reliable pet because unlike Heimer, hers stay around regardless of distance, and unlike Malz and Annie, hers don't spend 75% of the fight running back and forth between enemy and master. As a side-effect of her turret style, she is capable of landing multiple continuous hits from her ability unlike a burst mage via her autonomous turrets.
For that, the ability to set multiple plant turrets is without contest, THE thing that defines her. Therefore her Passive should either play on that, facilitates that, or somehow flows well with that play-style.
- Option 1 - This one plays straight into the fact that she can lay an endless unstinting assault upon the enemy
Bramble Fungus: All base damage and maybe even ratios nerfed. Her Passive now inflicts bonus magic damage upon all auto-attack and plant turret attacks and applies a bramble stack. Her Passive's magic damage increases based on the number of stacks on her target. While the base damage is meh, it has surprisingly high ratio making it necessary for Zyra to farm up for AP.
Thematically speaking, roses are notorious for the fungus it holds that messes your shit up, hence why you must wear gloves when handling roses, ALWAYS. Gameplaywise, it plays directly to the fact that Zyra is capable of constantly inflicting small ticks of damage during the entire fight, this isn't 100% unique to Zyra, but it fits her playstyle AND it fits her thematically unlike how it would be on other champions. This Passive will also rely on AP scaling rather than bases.
This way if Zyra goes Support, she's there solely for her CC. But if she wants to also deal damage, either she's going to be stealing a whole lotta farm and kills, or she's going to have to midlane for farm.
- Option 2 - You want to keep the stupid death Passive? Fine.
Floral Reincarnation: When Zyra dies, the player have X seconds to select a plant or seed that is on the field. That seed begins to morph into a bud with half of Zyra's hp and resistances. After Y seconds if the bud is still alive, Zyra will revive out of that bud. Like any revival Passive, this will likely have a cooldown too. The bud will be revealed for all players to see even if it's in a bush. If there are no plants or seeds nearby, Zyra just dies.
I'm not that big a fan of this one because it does literally nothing to help Zyra become an AP mid again over being a Support. However I consider it an improvement simply because it fits her both thematically and plays into her plant summoning playstyle. Often times putting plants close to an enemy is preferred for damage, with this Passive, the player chooses between a safer revival or more damage.
- Option 3 - This one forces players to choose between staying safe or risking themselves for more effective damage
Botanical Bond: Zyra and her plants gains some sorta power-up based on how close they are to each other. Whatever the power-up is, it should be enough to be worth risking Zyra's butt to get so close to the danger-zone. Also the duration of the plants are prolonged if Zyra is nearby.
As explained before, Zyra's plants are often placed at pretty close to the enemy while the slow and fragile Zyra stays further back. This Passive rewards Zyra for risking her green butt. She can still safely harass at a distance, however it is less effective than if she bring herself to close to the vicinity to her plants. This is especially risky for Support Zyra who have Leona's, Sona's, Thresh's, and the enemy ADC to watch out for. So if they only try to safely poke, they will find their harass a lot less effective.
And like a previous one before, it plays directly into her unique niche that most other champions do not fill.
Personally speaking, Option 1 is the simplest and probably the best of the 3.