Ok i might throw some ideas:
- First thought came that sphere can consist of spherified cube; say a cube with NxN grid on each of 6 sides and vertices moved so they are from equal length out of center. This divides sphere more evenly into "tiles". Each side is 90 degrees away from each other.
- Day night calculation. 2 parameters needed: Earth rotation angle, Earth angle towards sun.
- Positioning, i'd place every object using 3D coordinates instead of 2D even when it's surface of sphere. That way it is easier to calculate distances and pathing. Note that paths still aren't linear; earth isn't flat Path creates an arc instead of line.. not bothered to draw it, formula might be easy to make.
- Zones/continents could propably be made of arrays of 3D points forming the outlines. Bit tricky checking if object is in area. Only other choice i can think of would be marking each "tile" with zone info, however this is very rough solution. Even as rough solution it could still be used as pre-calculation, narrowing down the choices quickly.

PS. Please somebody make a Starcraft like 3D game that features several planet like battlefields