In game design there are few answers. There are however two very good questions:
Is it an excellent game?
If you are unable to answer the question early, you are taking a huge risk. Risk in any business is exciting and can pay off immensely, but few afford to fail more than once.
Here comes the hard part; the question is not "Can it become... " or "Is it a good idea..." Is it a good game RIGHT NOW? If you keep on answering these two questions during development until the answer to the first question is "Yes", you barely need a game design document and more importantly, are not wasting a lot of energy due to speculation. Which, by the way, a game design document is. Speculation. Speculation is fun, but not very useful.
When the answer to the first question is answered with a great "YES!", ship the game, get fame, moniez and ladiez, repeat.
And so we can pretend this is an art related blog; here is a discarded sketch for the miniatures game: