Zilean mid is actually pretty strong, I've played him in ARAM a couple times and with 40% CDR your W (which reduces all your CD's by 10s) will be on a ~3.5s CD = Perma bombs, slows, speed ups and an insanely low CD on your ult = tons of utility and dmg
The only reason I don't play him is that he is very mana hungry and his lasthitting in lane is crappy
His teamfighting isn't as weak as people might think as long as you stay back and put all your stuff on the frontline and let them do stuff. That's probably why he is played more as a support nowadays