Zilean rework
Zilean is in an odd place. With only 1 damaging spell he still needs to build AP for his ult to be effective, with only 3 real spells and 1 of them seems to be tacked on so he has damage.
Here is my rework for Zilean, the Chonokeeper
Passive - Time lock - Whenever Zilean takes damage from a champion if he isn't hit again for a time, the last hit is erased. - The idea behind this passive is Zilean is naturally squishy, and should be building AP not tanky stats to be effective. This allows Zilean to get hit, and if a few seconds pass without him being hit again, the damage will be undone. Great against that Graves ult that hit you, or the Ziggs bomb, useless against any DOT effects that might hit.
Q - Paradox Shift - Target enemy experiences future pain and past injuries all at once. For the next few seconds increases all damage done to the target, then explodes for bonus damage based on damage done. - Now this is a time bomb. Click a target, have your team unload, and BOOM, they get hammered for a bonus. The base damage would scale with level, as would a bonus cap.
W - Flux capacitor - Zilean drops behind a small device that charges over the next few seconds, once charged Zilean may recast it to teleport back to the device and leave a small explosion at his prior location. Damage done to the device can destroy it. Flux Capacitor grants vision. - We see Zilean in the lore as an inventor, but we don't see this in game play. This gives him both a tool to use, and a reminder than he is more than a mage. It requires forethought on placement because the device is fragile but it can also work as a distraction to delay his foes who chose to destroy the device rather than chain Zilean more.
E - Time Warp - Now a ground area skill shot, Time Warp leaves behind a vortex that attaches to the first champion to walk over it. It speeds up allies and slows down foes in the area. If cast on the Flux Capacitor, greatly lowers the charge time. - Honestly, you could keep the prior E, but I feel this one allows for more counter play than a single target slow/ haste. Haste an ally and others near him, slow a foe and other around him.
R - Chronoshift - Too iconic to change, too unique to remove. Rebalaced for his current kit. I love Chronoshift and couldn't imagine a better ult.
Lane - Zilean should be support but in unique cases may go mid or even top! Against low harass top lanes or mid lanes Zilean can poke, trade, and back out to restore some of the damage, against mages with longer cool downs Zilean would be a strong counter pick. Yet as a support he can really shine. His passive allows him to take a little damage in trades and restore it while giving allies boosted damage, his flux capacitor lets him back out after a trade or prepare it before diving for an escape, his time warp lets him help during jungle ganks or assist his Marksman in getting that last hit. Finally his ult lets him back things up should the tables turn.
Teamfights - Everything Zilean does in lane, he does better in team fights. More damage to amp up with Paradox Shift, and more allies and foes to speed up and slow down. Flux capacitor still lets him back out of things turn south, or drop it over a jungle entrance to watch for foes that might walk by it, and then move to intercept or keep the jungler from joining the fight.
The key to playing and winning with Zilean is forethought. Dropping time warps to create choke points, using Flux Capacitor to jump out or in rare cases into a fight, and being able to ult his carry and keep them from being bursted down.