This topic is my jam. Here we go:
Can totally work as a support. Put points into traps, and use her crazy range to poke them down. Harass with her auto attack range and keep them from initiating.
So underrated! Cleaver pressure is absolutely terrifying, and he can scale decently well into mid game! If your looking for a tanky support, go Mundo. Remember that early game he's not as tanky as he should be, and he needs some items to start scaling, so play safe and throw LOTS of cleavers to harass.
It's very hard to deal with his AD burst if you build him right. Can be funny to run a 'kill lane' with.
The blind is a great tool to stop the enemy ADC from putting damage on the board, and can work decently well as a cc. Mushrooms are very, very annoying and anti-gank potential with how much attention bot gets.
So, so good as a support if you know what you are doing. he's got a great stun, and some of the most annoying, unending poke in league. If you are good with is Q, you can stop the marksman from farming pretty well.
Build AP/Health and max his AoE ability, it has one of the best spell-damage returns in the game. His ultimate is TERRIFYING at level 6 when the enemy attempts to start a fight, ult your ADC and turn it into a 3v2. Though after his rework, this will not be possible :(
Zilean is a monster right now, and in my opinion one of the best supports/mids in the game.