I feel like this character is pretty powerful and fun to play in dominion. Given that people like to play high-output characters like Darius/Kha'Zix etc, he should fit into blind pick comps nicely. Unless your team has no-offence characters.
What I noticed: Blackfire Torch changes made it surprisingly good item for Zilean. You can start off with boots+Fiendish Codex, which are later built into Sorcs and BFT. Why has it become good? 20% cdr - way to go. Mana regen? Gladly! AP and passive to improve our bombs damage? Perfect.
I also noticed that 9-0-21 should work relatively well; you want that 10% cdr at start and initial mana/regen bonuses, as well as movespeed and cdr on summoners (-10% cd on revive, haha). You shouldn't try to 1v1 bruisers, obviously. Also, with dominion's manaregen aura, manaregen from BFT and masteries, you probably won't ever have mana troubles.
Any ideas on what to build later? Or maybe something entirely different? I thought Hextech Sweeper should be way to go. Sweeper caps CDR at 40%, gives insane MS boost and more utility. Also hp/ap. Maybe Wooglet's later.
I wonder about Tear, Zilean can stack it pretty quickly and large ap boost from Seraph's.