Zilean's issues
New Zilean rework is pretty neat as a support change, he have tons of tools and options, and he became a very unique character. The issue is that his bombs became very clunky, especially the double-cast.
There are a few issues with his bombs as they are right now.
A. Until you get level 5, QWQ combo forces you to wait on cooldown of Q, this makes him very clunky to play. B. You can't blow up a bomb on the ground by throwing a bomb on it.
Overall those two things makes Zilean unnecessary clunky. His kit is already quite difficult to utilize, and his QWQ stun-combo is hard to properly pull off, and if you miss it, you are in a bad spot. His ulti require good timing and communication to do. His E is so multipurpose it's crazy. All of this leads to a very hard to use kit. What I suggest is that level 1 Q, have 10 sec CD instead, and that you can blow up a grounded bomb as well, this will give zilean better zone-control, and overall be a buff to him, but I don't think it's too much of an issue to buff him a bit.
Another thing is that his double-cast seems to be high risk medium reward. You need to properly hit 2 skillshots and use LOADS of mana(roughly half your mana at level 2), to do what some champs does with a simple point and click. Range is not spectacular, and it's not very easy to hit both, and even then when you DO hit, all you is rewarded is a short stun. It's a creative ability as you can use it on your ADC/initiator and then follow up with another bomb as a follow-up stun or as peel, but overall it's a very difficult ability to properly use, and I feel the AoE is hard to appreciate.