There are three basic ways to achieve portability:
1) Create a system that meshes (more or less seamlessly) with the existing system. This is the 'promise' of 'standard' D20 and the OGL, although various verisons live up to it more or less well.
2) Create a system using roughly the same dice engine and the same base structures, but running along side rather than merged with the existing game. For example, you might add some skills and a new resource type for your mini-games, but avoid changing things like combat and the class structure.
3) Create a seperate system that handles something simply not covered by the basic rules. You can use whatever mechanisms you like, so long as you do not screw around with something in the core system.
Most of the time, I'm forced to go with option 3. Option is also interesting, but somewhat more complex, especially when you want to pass influences into a core system.
The OGL books generally use option 1, but that creates varying degrees of conflict between the 'merged' rules and the 'standard' rules.
Shannon
Mongoose
Current Status: Treating shackle gall