Design documents are good but don't work for everybody or everything you create, perhaps you could try using one on your next project & see how well it works for you?. It's all to easy to look back & think "I should have thought of this xxxx feature at the start" but it's natural that extra ideas pop into your head while you are creating something, be it a game or art.
How many times have you modelled something in lightwave etc & then thought "Hmm, if I add this to it or tweak that it would be better?". Programming is no different in that there is nearly always something that you think you can add/remove or change, even weeks or months after the project is finished something pops in your head like "I should have added blah blah blah".
A lot of the time the hardest part is when to say "Enough is enough, I have all the things implemented that the project requires so time to tidy up & finish it"
I'm envious of you in a lot of ways in that at least you have got something to show & works, whereas I have got more code snippets & routines that do a specific job but no "Product" so to speak . Truth be told I have completed a few things but none of them are games & were asked for so I had in easy in that people came to me with what they wanted & I done it, if anything was missing that was down to them as they were the client & I created what they asked for.
Keep up the good work Erico & look forward to the final result