I think I found out what makes Zilean seem "weak"

People are playing him wrong. The build that I go is I start Q,W, E and of course R at 6 but for your maxing order max R, E, W, Q. Than go for this kind of item build I like to go of course since your support and you need wards and get this to get around fast . Rush this baby but start with its baby because your playing Zilean support for his utility not his damage and your laning is weak so its better to start with coin than with since you shouldn't put yourself at risk to poke with Q since u are very squishy. Than get this item because it gives you nice armor and sweet sweet cdr and makes your adc a god. Get next for those pesky aps and to help split for your team because Banner is crazy strong for fast moving supports like zilean and bard. Last item can really be a tank item like or if you want utility try this . With this Build you should have 40 cdr and with 40 cdr your ultimate is on a 39 SECOND CD yes its that low with 40 cdr. Now you might be wondering why I don't Max Q first its because the slow is super super good since the slow and speed is very strong and you can just slow a enemy and be able to do a dbl q stun a lot more easier and with maxing w second is very good to get your spells off cds fast enough and q is more or less just used for its stun not its damage that a lot of people are doing. People keep building ap items on Zilean when it's simply not good compared to ap supports like . Zilean is not like those supports he has very great peel for his team and can help get them out of jams that just make full ap support zilean terrible in comparison. Zilean is best played as a utility support like because it puts his kit to his full potential the ability to cast multiple E's and getting off his dbl q stun is very strong compared to ap zilean which is better off playing mid.

TLDR: New is sleeper op because with 40 cdr is absolutely disgustingly strong on him and is better played more utility rather than damage. The only thing he really needs is either a buff to his passive or a new one.