Great game, list of possible additions:
A switch to prevent you from accidentally spending gems. Which is extremly annoying and would stop me from playing altogether.
The real life cost of some of the gem animals is off. The panda, mammoth, dragon and coala are to expansive, half their costs now would still be expansive but closer to achievable.
A way to turn of the icons above the attractions and animals. To watch the zoo without those things distracting you.
I'll add more ideas later.