Recipe: how to create a game

I actually have no idea what the recipe is, but I'm hoping that if I keep designing games I'll start to see patterns in what does and doesn't work. 

So far I have the following things a game needs:

• A clear end point:  If the game doesn't have a goal or a winning state to work towards, then it's hard to find the motivation to keep playing.  This doesn't necessarily mean that one player beats the others, it could mean that the whole group achieves the aim, but there has to be a clear moment when the students can say "finished teacher!" and prove it.
• To be replayable: The essence of learning is repetition and the best games make you want to play over andf over again
• To involve creative decision making: a game is usually replayable when there are many possible ways it can end, which means that every turn should see a player making a creavtive decision based on many possible options available. 
• To be playable for multiple levels of learners at once: since any group of EFL learners is by definition going to be composed of students with different strengths and weaknesses, a game will only really be possible if both strong and weak can play enjoyably and successfully together.  Adding the element of creative decision-making will help with this because each player can make choices based on their own level of understanding.
• To have simple instructions: If the instructions are so complicated the players don't know what to do, the game can't be played no matter how brilliant the original design.

Any other key points?