Any idea is a good idea and at least needs to be written down
Once an idea evolves into a potential game, we write up an internal proposal detailing what the game will be about and why we should pursue it.
Then using anything we have on hand, we'll rough out a design to get it to the table as soon as possible... even if it's just piece by piece.
Finally, we like to get a printed copy of the prototype to see how it would look and feel in production. We realize this might see like a needless expense, but it's important to us.