Whatever happened to people asking for BlueCube to help?

Anyways, you could have the sprite start at 100% in various areas (hunger, health, thirst, what have you) and have them go down by a certain % every once in awhile, or when a specific action takes place (happiness goes up when you pet the animal, for instance).

For the actual actions, you'd just have to make your own sprites for them (petting, feeding, etc.) and have the sprites change when you are doing one of these actions.

It won't be easy, and I don't think you'd get the same level of depth as Tomogachi.