My opinion on Zilean
To start off, I am not a Zilean main. I don't even main his usual roles. But I used to play him a lot, and all the problems (the bugs his abilities have) I saw when I played him all the time back in July of 2018 are still present. This is also entirely opinionated and just some thoughts I had on how to make him more fun and tilt-proof. Last little bit of info before I start- this is for zilean as a support. I do not know, for I have never done this, how any changes like these would affect zilean in the mid lane.
His passive: Current: Zilean's passive is one of the stronger ones out of a support champ. Being able to level up an ally during a fight, especially during an early bot lane fight can turn the entire fight on its head. A level 2 Ez gets a health boost and his 3rd ability with level 3 halfway through an early game gank? Instant win of the fight. But the downfall to his passive is apparent and massive- it takes TEN seconds out of combat before you can use it. TEN. The majority of a fight will be over in the first 10 seconds, so his passives only real use is to gain a slight advantage before a fight. Don't get me wrong, this is still good, but it could use some improvement. Improvement: while I like the time out of combat before his passive can be used, 10 seconds is a lot. If you bumped it down to 5 seconds or even just 8, I believe that would still be fair. With other incredibly strong supports such as pyke, Leona, and thresh dominating bot lane, a simple buff to Zilean's passive would help with making him more viable. Improvement 2: if Riot is happy with the 10 seconds, then maybe change it to 15 second's, but allow his W to work with the passive. If he pressed W, it could decrease the 10 seconds it does off the 15 second's of the passive. This would make in combat level ups strong, but with the cost of Mana and a W.
His Q/W Current: Zilean's main damage is obviously his Q + W + Q combo. Hitting all of it will result in heavy damage and a reasonably long stun that is rewarding for someone who just hit two skill shots in quick succession. And although I feel his Q and W are well balanced, there are a HEAVY abundance of glitches associated with them. Glitch 1: the most simple of these glitches is the speed at which the abilities can be cast. When you cast your W too quickly after you throw a Q bomb, there is about a 25% chance you won't get your Q cool down reduced. The game stops you from using your W if nothing is on cool down, so it's not possible to accidentally hit your W before you launch your first Q. This is a glitch I've been waiting to see in patch notes for some time, but I haven't seen anything. Glitch 2: the basic Q hit box can be a little gltichy at times. There are several Q's I've hit that I know I shouldn't have, but that may be champion hit boxes more so than his Q hit box. Glitch 3: this glitch is one where I don't know if it's a glitch, or purely a visual mistake by the game. Zilean's Q prioritizes champions, so if a champ is in its stick zone with 3 minions, it will stick to the champ. Yet sometimes, if the champion enters the stick zone very quickly after it hits and locks on a minion, the bomb will swap targets. I have seen a bomb go from ground to minion to champion in a matter of about .3 seconds, which is why I feel this is just the game trying to lock onto a minion but then telling itself "no we prioritize this instead."
His E Current: for a champ based around time, his E makes complete sense. But there is a clear problem with it: point and click CC. Riot has been slowly revolving out point and click CC, but zilean remains unchanged. It is very tilting to lose a fight because of a single button press that slows you the extreme amount that it does. And it is unstatisfying as zilean, too. In my opinion, I feel I didn't deserve a kill I got because I won after a single button press with very limited skill. Improvement: I've been trying to figure out how to change his E for some time now. But in S8, Ez got a rework and it has a mechanic that I think could work well with Zilean. I forget which ability does this (never played the reworked Ez) but one of his abilities will ignore minions and hit a champ already hit with his W. This is an interesting concept that could help Zilean as well. Instead of a point and click E, we could have a small little projectile that hits enemies or allies to slow or speed, respectively. Yet you could still speed yourself, by clicking on Zilean like you still do. But this projectile would ignore minions and anything his E can't hit in it's current form.
His R Current: I like his R where it stands now. If they changed much of his current kit, his R is pretty balanced as is. Improvement: the only thing I could possibly say about his R, is to not allow allied champs to use Zhonya's or Guardian Angel while Zilean's R is on them. I used to see allies all the time activate Zhonya's soon after I used my R on them because they hadn't realized it yet. Just a small quality life improvement.
Finally: give the man a new skin, it's been so long. I recommend something on the lines of making him a restaurant worker, with his clock being a giant hamburger. Completely silly, but it may be able to work haha.