After playing with a pru zilean I've come to the conclusion
If this dude had a better passive he would honestly be a very decent and good champion, especially strong in this current meta of assassins and bruisers.
The patch notes once said during the tank meta that if Caitlyn wasn't a viable pick in the meta of kiting and range, then something is wrong. I say if we're in the middle of an assassin meta and zilean isn't a viable/popular pick, then something is wrong with him.
We could easily change his passive to give 10% CDR in an aura to allies and have him be a good pick. For example.