>I went full waterfall on it, the spec is 266 pages long and took half a year just to write.

Wow. It's nice to see extensive planning like that. I usually just throw anything that seems like an important idea into a text file on the assumption that it'll be fully hashed out during production at a later time. That method entails far less work, with the caveat of occasionally wondering if you're delusional from time to time.

In any case, good luck. I hope you kick ass. If Microsoft employs their usual design aesthetic, doing so should be very possible.