I have played zilean for around two weeks now as a mid in blind pick, and it seems to me like he is viable, yet not as strong as other aps other than in utility. I think this could be changed and also make him a somewhat okay support. I think that if they changed his time warp from
Time Warp -Zilean increases an allied champion's Movement Speed, or slows an enemy champion, by 55% for 2.5/3.25/4/4.75/5.5 seconds.
to being
Time Warp-Zilean increases an allied champion's Movement Speed, or slows an enemy champion as well as heals/damages the target for an amount thought of by riot and speeds/slows the target by 40% for 2/3/3.75/4.5/5 seconds with the heal/damage increasing per level.
This idea is just a thought as to how to make him stronger in general and allow him to have be better than he is currently in ways that would improve general strength, yet not make him op by making his 55% move speed come with a side effect, since that works for nasus and champs like that, but it would be too much for zilean since time warp is mainly for utility at the moment, and is one of the stronger slow/hastes in the game, so it would be a bit op if it still held the same slow/haste but had a damage/heal side effect. Just a suggestion.

if zilean healed he would be a game-changer. This would be far too much utility and sustain for one support, IMO.

This is an unlikely change, considering Riot's (Morello's) stance on healing. Also, Zilean already has a form of healing- negating a champion's death. His ultimate is always useful, while a heal would be most important during laning phase (unless that heal had a good AP ratio). He currently has a movespeed booster and a good harassment tool (both of which can be used twice) for laning (and they remain very useful later on). His main issues are that he's mana hungry, is better with AP than with gold-generation items and their upgrades, and his bombs steal cs. He could also use another skin.

i agree with your assessment of zil apart from 'stealing cs', that's not an issue. a early game zil with good items is much more dangerous than the ad. Its this ilogical concept of demanding cs from the heroes that would benefit more from the money early game. pardon my dota lineage but 'adc' are late game heroes. delibrately gimping your early game for the 'adc' is a flawed strategy.

e.g. ap owns the lane. your ad may not have gotten the kills but the opposing ad is in the same shoe. except you now have ad and ap. early game hero simply need to transistion ease off on the hero ks to handover to late game heroes.