What about this system for spheres?
Spheres:
Every God will have to choose a main sphere. (It'd be nice for it to be something that could be divided into smaller spheres. For example: Life, which could be divided into nature, birth, and lots of other things.)
Next they will choose a second sphere: If this sphere would be either close to, or some sub-sphere of the main sphere, you would get a third sphere. (If you'd choose birth, you'd get yet another sphere. If you'd choose growth, you'd get another sphere. If you'd choose Earth, you won't get another sphere.)
The third sphere will always be a sphere that is close to the main sphere. (For life it could be nature, for example. This to make sure you won't choose Life - Growth - Death, ending with a totally different sphere.)
Whether spheres are close to or opposed to one another is open for discussion.
Your acts are more powerful (or cheaper) when they overlap with your spheres. When they oppose one them (God of Life raising undead) they will be far more costly and less powerful. (Exception if you've got only two spheres that oppose one another: Life <-> Death). If the aforementioned God of Life, Growth and Nature would 'boost' the forests to grow, they would use all three spheres, and it'll have a massive effect compared to a God of for example Fire would have.
Note: If you don't like this system, we could always brainstorm for another one.