Design documents are a pain to create but are the best references that designers can create for the team. I’d recommend to take at least a month and write down every little design aspect. The best documents are hundreds of pages long!
During this phase you should be able to tell whether or not your game is too ambitious for your team. If it is try to cut out gameplay features. There’s always room to add those in for a sequel. 😉
With The Divine, we built the engine before the game itself was fully designed. We knew that we wanted a space shooting game but we didn’t plan to put in special effects and the sort. Inadvertantly the coding is a little messy and locating a bug is becoming rather difficult. Now if we knew ahead of time every little aspect that we would need in-game we would have been able to write a more efficient engine that would suit our needs. But oh well we’ve learned from our mistake. Make sure that you do too.