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.

Starting codex makes you fairly weak at the windmill fight. They're removing the mana regen from the codex next patch anyways. Zilean being so squishy I'd think you'd want some HP for the initial fights and so that you don't instantly melt to a burst champion.

I found that starting with Catalyst into RoA and getting CDR boots to be quite helpful. Otherwise he is too squishy.

I'm a big fan of Early Mpen Mage transitioning into tanky Support. The base damage on the bombs is fairly high(320), but that's his only damage ability so he has trouble scaling into the late game with it. I'm not a fan of RoA as well, since that gives up his early game presence for some mid game tankiness, but HaIfhearted and MMKH seem to like it.

POOTING!!!!
Lol ok sorry I try woglets early for holding towers but doing this gives you the worst early game ever. But with wooglets active and zileans ult it is definitely harder to kill him and gives you ample time to stall for your team to show up. I think this could become a late game item though instead of rushing it. Like you I am looking for the optimal build for zilean.

Zilean is pretty mana-intensive, so I like RoA on him just so that he can keep spamming.
Otherwise I simply go oom too quickly when I want to cast spells.

However, I prefer to grab the Blasting Wand first when I'm going for RoA, and I start with Prospectors Ring on him for that extra early-game kick.
Then I go RoA > Ionians like MMKH does, followed by Chalice/Glacial and Void Staff.

the best zilean i saw built athenes, iceborngauntlet, boots of lucidity, and a fourth item, i think bft or wooglets. He was perma slow for our team, permba buff for the enemy team, and they rolled us.

im gunna say his fourth item would be wooglets. he already had the cdr from boots, ff, athenes.