Zilean is one of the least played champions in the game. He has no certain role that he's very good at Iike Nidalee but unlike her he isn't good at any role it seems.
As a support Zilean feels lackluster even with the massive bomb poke your speed boost/slow and a possibly game-changing ultimate isn 't what a support needs. I am not contradicting with myself. While the ultimate is a very strong ultimate for a support type of hero it needs AP to heal a good portion of your HP, without it Zilean's ultimate will act like a GA which is still good but not as good as a full revive. The speed boost is massive the slow is very strong too but it's not going to change the game too much. When you take other support heroes they provide more utility than Zilean does. Sona brings passive powers with her auras and the active components are pretty useful aswell. Janna brings disengage power with powerful shields and a panic button on R.
As an AP mid Zilean certainly has good damage, but it's mostly single targetted. Another problem is he plays as a support lategame eitherway.
With his current kit I'd say Zilean excels at setting up a kill with a good nuke from double bomb and a nice slow/speed boost to back it up with a revive to save someone who's overextending, got caught.
I'll go over his skills quickly and explain how I explain the uses.
Q - Time Bomb
Puts a bomb which does 6?(10?) true damage then explodes dealing damage in an area around the target
General: It's Zilean's primary source of damage and it will be what you use for damage.
As a support: Time Bomb is a very good ability for harassing early game but it might push the lane if the enemy can detonate it near their minions therefore pushing the lane. One of the better uses would be use it on a minion that's low HP and close to an enemy champion and have your ADC instantly lasthit it or rewind and put another bomb on that minion. You'll get an instant double bomb detonation in an area around that minion possibly damaging 2 champions instead of 1 if they were so clustered. If you use this on a champion they'll keep distance from the other champion. Late game the damage falls off.
As a AP mid: Only source of damage. You use it for strong nukes and to bully the enemy midlaner till you reach level 6 (Godlike status) This spell doesn't fall off late so you don't have to have it as an useless spell.
W - Rewind
Refreshes every spell's cooldown by 10 seconds(excluding Rewind for obv. reasons)
General: Most of the time you'll use it for the DTB(Double Time Bomb) but if you have enough CDR and maxed out Time Warp you can use it for permanent speed boosts. (Or until you kill your mana) It has the same uses as a support and AP mid. You should spam this spell whenever your ultimate's on cooldown.
E - Time Warp
Slows target enemy or hastes target ally.
General: Proboably the spell with the most uses on Zilean's kit. You can use it to escape/save an ally/set up a kill/slow a chaser and more.
As a support: You'll mostly use this to get back to lane faster early game. In the lane this spell does wonders, it's much better to haste an ally than slow an enemy most of the time because if the enemies have gapclosers they'll still get onto your carry even if slowed. Another use will be on yourself to get in range to harass or on an enemy to set up an easier kill.
As an AP mid: Haste yourself to get in range to put DTB and back off. Also you'll most likely use it to escape from jungler ganks or to haste your jungler/slow their mid when your jungler's ganking. As for late game if you have stacked up enough mana you'll most likely spam this spell whenever you can.
R - Chronoshift?
Marks an ally champion for a short period of time. If the marked ally dies they'll be revived with a certain amount of HP.
General: Gamebreaking ultimate giving a second chance to your allies or yourself. Also one of the primary reasons midlane Zilean rarely dies post 6 and the reason he usually pokes you out of lane.
As a support: You'll use it to revive your allies most of the time or if you're the one getting focused and your ally isn't in danger of dying you'll use it to save yourself. But you'll most likely sacrifice yourself for the ally.
As AP mid: Makes you hard to kill in lane as you can pop this when you're sure you'll get killed and if you don't just suicide to the tower or run away. You'll still use this on allies and it will give your AD carry 2 lives.
Now I pretty much pointed out the good things.. The bad things are that this champion can't carry as an AP mid and becomes a support later in the game. Well he can dominate his lane but with enough time and MR enemies will stop taking 70% off their HP from DTB. And the longer the game lasts the more support you'll become. As for the support problem is you won't provide much early game other than harass and a escape vs ganks. You'll provide poke but no sustain/shields/DPS, while late game you'll become a beastly support you might have mana problems, your ultimate won't heal too much, Time Bombs will just be a random spell for some damage.
I think this champion deserves a rework because he should be oriented towards one role.
Now I'll make 2 sorts of reworks: a Support and a AP mid one.
Passive - Time Rift
Zilean's attacks on enemy champions refresh his cooldowns by 2 seconds (2 second cooldown)
Q - Time Bomb
Remains the same, but increase the base damage by 10 for all levels and increase cooldown to 12
W - Chronoshield
Zilean puts a 80//120/160/200/240(+0.5AP) shield on target ally. If the shield is destroyed Zilean gets his cooldowns refreshed by 2 seconds. The shield lasts 5 seconds.
Mana cost: 90 at all levels
E - Time Warp
Zilean hastes an ally by 55% for 2/2.5/3/3.5/4 seconds if an enemy attacks the hasted ally their attack speed is lowered by 10/15/20/25/30% for 4 seconds.
Mana Cost: 90/95/100/105/110
R - Chronoshift
Zilean marks target ally for 10 seconds. If the target ally dies under the mark. It's consumed and revived with 750/1050/1350HP and deal 10% bonus damage for 5 seconds.
Mana Cost: 150/200/250
The reason behind the passive is to encourage attacking to lower cooldowns and attack poke. I gave Zilean moderate cooldowns because with the passive you don't want all your skills to be ready since that'd be a bit too OP. I believe one major problem I might have made was with the numbers so excuse me if you think those are too high. I added a nice little touch to every ability except for Time Bomb. Chronoshield will refresh Zilean's cooldowns by 2 seconds if the shield's broken letting him use his other supportive abilities faster if they're on cooldown. Time Warp gets an attack speed slow, I think the usage should be to help an ally escape but you can also use it in a skirmish to reduce the enemy DPS with their autoattacks. As for the ultimate I added random 10% bonus damage because most of the time they'll be surrounded by enemy champions and that 10% bonus damage might come in useful if they're not.
AP Carry Zilean:
Passive : Splitsecond
Everytime Zilean uses an ability his other cooldowns are reduced by 1 second.
Q - Time Bomb
Same skill but reduce AP Ratio to (0.7)
W - Cursed Time
Zilean curses target area dealing damage over time to enemies hit while reducing their speed by 10% every second as long as the effect lasts.
Duration: 5 seconds
Mana Cost: 70/80/90/100/110
Cooldown: 21/19/17/15/13(basically with the DoT the cooldown is 16/14/12/10/8)
E - Seconds
Passive: Zilean gains bonus 1/2/3/4% CDR every time he kills an enemy champion (obviously cap is 40%)
Active: Zilean stuns an enemy dealing 60/110/160/210/260(+0.5AP) damage and dealing 50% of the damage to enemies in 300 radius of the initial target after 3 seconds.
Stun duration: 0.5/0.75/1/1.25/1.5 seconds
Mana cost: 85/95/105/115/125
CD: 13 seconds
R - Chronoshift
Boom! AP Mid Zilean has 3 damaging spells he has some CC but he's still that single target magic damage monster. His ultimate still provides that hard to kill post 6 status and now Zilean has some nice damage spells. Time Bomb is most likely still your main focus as with it's high scaling and good base damage you can bully enemies. The new change with Zilean is that most of his damage is delayed. He's more of the hero that gets kills later in the fights instead of bursting instantly like LeBlanc/Annie. The delayed damage might prove troublesome against carries since they can lifesteal it but at least Zilean has more sources of damage. The increasing slow on Cursed Time can be used both defensively and offensively while the stun on Seconds allows you to land your abilities easier while dealing a good amount of damage. The reason I made Zilean a targetted mage is because I feel like we're running out of those. Annie/Ryze are one of those mages that are pretty reliable and I want Zilean's skills to be reliable.
These are just my ideas, my opinion might differ from yours so I suggest you guys create your own reworks/post reasons why Zilean doesn't need reworks/review my rework(s)/play more Zilean over the most played Caitlyn ^^