In today’s RPG marketplace, with the advent of OGL, wouldn’t it make more sense to simply use an existing engine from something like Pathfinder, Savage Worlds, or Fate? Wouldn’t a proven system with a nice track record, and designed by folks with vast amounts of gaming design experience, be a better way to go than trying to cobble your own mechanic together?Rules and mechanics matter. What do you want your players (not the characters, the players) to do in your game? Spend lots of out-of-game time designing characters that they then unleash in a handful of set-piece battles per session? Then Pathfinder’s probably the way to go. If the players are going to be organizing raids in which their characters lead groups of NPCs in disparate (and possibly desperate) missions, Savage Worlds is clearly the way to go. On the third hand, Fate would be an excellent choice for a rules-light, fast paced game that rarely gets bogged down in either tactical or rules-interaction minutiae.
Designers need to keep in mind that rolling dice is not playing the game. Rolling dice is where the play stops and everyone waits to see how random chance will rearrange the situation, at which point the players can start making decisions again. It’s this deciding that is the real meat of playing the game. That’s where the fun comes from. So what sorts of decisions do you want your players to be making? Design your mechanics to support (but not supplant) those decisions and you’re off to the races.