I would make a Tarantula keeping sim, which features 100s of the species available in the hobby. Kinect versions would allow you to use your body movements to manipulate cages (moving them around, turning them), play with (annoy) the tarantula and PC versions would have the same feature but with either Kinect PC or with the mouse, plus feature a internet feature which links to a website where you can chat to people playing the game at the same time to trade tips.
It would feature a fully fledged feeding system, which would allow you to purchase various insects, like roaches, crickets, locusts, phoenix worms et cetera, and allow you to breed them as well.
Tarantula breeding would play a big part. Some species will be easy to breed, others will require certain conditions like little or no light, flooding the cage, et cetera.
There will be various methods to acquire tarantulas, you can go to the local petshop, buy from a local breeder, or purchase over the internet. However, like in real life, shipping tarantulas in the game will be risky, like in real life.
Some species will be easy to acquire, other's wont. Some will require months of waiting to acquire.
You will be able to upgrade your shelf size, buy bigger cages, experiment with various substrates and keep certain other inverts as well, like Emperor Scorpions and Centipedes.
What stock stores and online dealers have will be randomised. Your character will also need to go to work during the day so you can buy more tarantulas or whatever.
You can make him work overtime or laze around at work, like the sims.
Cages can be decorated in a style similar to Sims 3, where you can drop bark, rocks, raise and lower substrate leves, create mounds, dig caves, et cetera.
If I had any Development skills, I would already start this project but yeah. The purpose of this game would be as a training tool, or if there are species to expensive or complex for you to keep, you can use this game to acquire them, where mistakes don't mean a loss of an expensive pet