I sometimes think of games as cars: they need to have an engine (the core mechanic), a steering wheel, gear shift and gas pedal (ways for the players to set their speed and direction), and the car body (flavor and theme). Also, if possible, a destination. The perfect game like a great car has all the parts fitting well together, and that’s what hopefully we are all trying to achieve.
— Andrea Chiarvesio

The Strategy Game Designer’s Constitution contains some “golden rules” for design, considerations for giving games strategic depth, plus a good list of potential end-game and victory conditions:

http://www.leiavoia.net/pages/docs/Strategy_Game_Designers_Constitution.pdf

Here’s the handy, pocket-sized Strategy Game Designer’s Cheatsheet:

http://www.leiavoia.net/pages/docs/Strategy_Game_Designers_Cheatsheet.pdf