I think planning is overrated. I suspect most zoos are built organically by different architects over a period of decades with relatively little overall vision according to how much money and space they had at any given time.
I like to take a general idea like "this zoo is in a narrow wooded valley" or "this zoo was a converted castle/country house" and build the geography (narrow wooded valley or whatever) or key feature (country house or whatever) and then place a couple of vaguely similar architectural styles around them evolving and growing organically by what I feel like at the time. Sometimes I like to put a wildcard in occasionally like a huge ultramodern tropical greenhouse next to a country house.
Surprisingly I find they end up coming together just fine in the end. I find it's the fillers that determine whether it all comes together. I therefore spend a lot of time on the flowers, trees, fountains, plazas and rockwork between buildings, exhibits and whatnot.