The problem with Zilean Riot
It isn't the fact that Zilean doesn't do enough damage, he does plenty. It isn't the fact that he has long cooldowns, his kit is designed to help with that. It isn't an issue with costing too much mana, just about every mana champ hurts for the first few levels. The problem is that you made him a skillshot mage, with ridiculous low range, and a slow spell speed. Maybe you fired the individual that designed Zilean, but from what I gather, he is a hit and run mage, hence how he was point and click before. Zilean is extremely squishy, and trying to stand their, and trade Q's with another support, mid, or ADC is murdering him. Do you know how hard to is to land 2 Q's, on any champ with a boot purchase that has a brain and understands to stay just a little bit back? It's not impossible, it's impractical. I seriously have to risk my champs life to attempt a double Q, because soon as I am in that range, it's worth an ADC to turn and attack me since I can be deleted in about 2-3 AA's.
Yes, I am aware he also has a speed buff, however when you are attempting to maneuver around minions and enemies to land a Q (not just 1, but 2!!) on someone HOPING in the giant cluster fuck that you got him; while thinking about buffing your movement, and spell rewind, while also retaining enough mana to do so and not get killed is a pretty ridiculous process. All of that to hopefully do some damage, AND land a small 1.25 second stun. That's a lot of work when a lot of champs can point and click that.
Riot, increase his bomb range, and the bombs speed. Put the mana cost of his Q back up. I don't care if it drains my mana, as long as I can be a safe distance and use it the way it is intended to use I am happy. I don't enjoy trading with Zilean, because he will never win a trade, and he's not designed to win trades.