A zoo theme has potential and TLG would have to be blind to simply ignore the possibility on account of 'it's not violent enough'.
The new horse mold is just begging for zebra printing...
The problem isn't that it's not violent; the problem is there's no conflict. With police and fire, you have a race to respond to various emergencies. Zookeeping, on the other hand, is largely routine feeding and medical check-ups. Mind, there doesn't necessarily need to be a conflict, as the farm theme from a few years back showed. But a theme that is built almost entirely around new molds would be hard to pitch without any sense of urgency. For this reason I think that a zoo will come when enough "zoo animals" have been created, not the other way around.
I'd reckon a circus theme might be more successful than a zoo theme, in any case. Not only would that require fewer new animals (lions and elephants might be enough), but there's a lot more action inherent in a circus than at a zoo. Perhaps once a few animals had appeared in a theme such as this, a zoo would be closer to becoming reality.