His Q feels really clunky, I really don't like that it stops zileans movement.
If his q didn't stop his movement, the same for orianna and karthus - we would probably see more of them in play.
Zileans passive is also. . kind of silly, I'm sure most players would rather mid zil than support him. Maybe a MR/AR shredding passive from bombing players? Wears down their armor over TIME!
His passive is fine, he's always had a passive that gave exp to people and it's a great passive. His passive initially gave all allies just passive extra experience gain, then only nearby allies, and now it's Time in a Bottle. His Q is also really fine. Almost every spell in the game causes the caster to stop movement.
Also, Orianna's Q doesn't cause her to stop movement at all, she's just extremely high skill cap. And Karthus is so good right now, it's just he's in a spot where he's been out of meta so long that people don't think he's good, but at the same time once you get your RoA, Tear, and Rylai's, it officially no longer matters if you're 10/0 or 0/10. His E is now a permanent slow field if enemies get too close, his Q now almost guarantees the next one to land if you landed the first one, and his ult is now a global slow and a bunch of damage. I've allowed teammates to catch up to enemies who were escaping with the ult, run right into the enemy team and died only to be a massive field of slow and damage to protect our ADC Jinx as she was gunning the entire enemy team down like the degenerates they were, and 1v1'd people who significantly outburst me just because they were too scared to turn and fight, but couldn't escape my E because they were under a permanent 20% slow. Karthus + Rylai's Crystal Scepter is honestly legitimately busted imo.
I would love to see Zilean changed a little bit but I doubt he will be. He's not popular enough and doesn't sell enough skins for Riot to consider doing anything to him and I say that as a 
main. He is nowhere near as powerful or impactful as he used to be before his rework.
"Point and click bombs are toxic" - "But hey, lets give 
one because she's an ADC and we like ADC's more"
Most players who go mid Zil are sheep who saw high elo players do it and decide it is more worthwhile than support Zil.
His Q stops movement? Preposterous! It's not like most other spells also stop movement upon casting or anything. Just because the rest of Zilean's abilities don't stop his movement, doesn't mean that his Q should get the same treatment.
Zilean passive might feel silly to you, but do you kno what it was before? Ambient exp generation. Though the one we have now fulfills the same purpose as before, it is now an active, which gives more satisfaction than just a normally unnoticeable aura.
An MR/AR shredding passive. Wears down their armor over TIME. I appreciate the fact that you want to improve the Zilean experience, but that just feels more like a bad pun in an attempt to convince your argument that he needs more power.
Gonna get real with you, here. I took the liberty of looking up your match history, and found that you only have one game of Zilean in your match history in the past month, which was also a loss. I don't think you have much experience playing Zilean beyond that one game, unless you have smurfs. Even then, I don't believe you have the experience with Zilean to make a valid claim of him needing buffs beyond that one loss which pushed you into making this one post clamoring for buffs on a champion you barely even played.
Ranting over. Now, I want to point out some things. Zilean's Q does 75-300 base damage with (90% AP) scaling, doubled with use of Rewind. The fact that it's also an AoE stun, it's more than a lot of supports can ask for. Though you might be thinking, "if he does that much damage, surely he's better off going mid where he can make use of it!"
900 range. There are so many other mid laners that can outpace his waveclear, zoning him from farming properly, and push him under tower. Zilean is not very good at last hitting under tower. The fantasy of making hardcore roams with Zilean is rendered moot if he can't keep up in farm as well. And while you can also place double bombs as mines for the enemy minion wave to come across, such waveclear is once again just a fantasy if the enemy mid laner pushes harder and zones Zilean off from the wave.
He only has one damaging ability. If Zilean can't land his bombs, how is he going to trade evenly with the enemy mid laner? Not only that, but Zilean is fragile. 499-1706 hp with 19.1 starting armor doesn't afford him any breathing room when attempting to make trades. If you can land them, good, you can keep the pressure on with the 10-8s base cd bombs. However, there are many mid champs which will make landing bombs a nightmare, like Yasuo, Zed, Ekko, Kassadin, etc. Not to mention, other long ranged mages won't grant you any leeway of getting within bombing range, like Lux, Ziggs, Xerath, etc. Unless they are incompetent, these champions WILL make mid Zilean's laning phase extremely difficult, giving him little to no chance at gaining a sizable lead.
While he, of course, doesn't particularly need to get ahead due to his nature as a utility champ, you're the MID LANER. The team is counting on you to do at least some form of reliable damage and waveclear, and if you can't pull through, all you'll get is a Loss and a bunch of flamers. If he goes support, he doesn't have to deal with any of that nonsense, and focus entirely on making plays in the botlane instead of also having to worry about farming waves.
The reason why he doesn't see more play is just like any other game with a roster of characters. You can't expect them all to have an even distribution of popularity; there will always be crowd/fan favorites. Yasuo and Lee Sin come to mind for never falling out of the spotlight. Characters like Zilean just sit back in their own tier of popularity, but at least it's not as low as someone like, say, Nunu.
