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.
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.