One of the problems with building a game like this in Google Earth is knowing what kind of surface you're on. It would be silly to have a wolf attack you in the ocean, or encounter a shark in Nebraska. I've solved this (and some other issues) with "zones".
A zone is a large area on the earth (the first one covers about five states in the southeast US). Each zone:
- Has a "level" to determine the difficulty.
- Is auto-populated with enemies of that difficulty level and zone type (land/water).
- Has a central city.
I think I might make it where you need to defeat a boss in the central city of each zone. Doing so earns you a prize (a crystal or something). Eventually, you want to defeat the bosses in every zone.