Make shit up on the fly. Its what I do. :V Besides, you're going to need to adapt. Just make a soft-plan.
Original? Now this intrigues me.
Unfortunately with everything being original it's FAR more dangerous to make shit up on the fly and not ruin the storyline... I've gotta at least have characters/quests/locations fairly planned.
Besides, my overly ambitious mind wants to put in a sequel so I can't go screwing up the story on the first part of it ;3
Basically a release date is "Whenever I get my ass to work on it" because I should, in theory, be able to finish most of the still needed planning within an hour or two. Enough to start it at least, and then I'll be pressured to do the rest in time :V
Also considering a shorter teaser thing to be released first that shouldn't take nearly as much effort to throw together, and to give me practice with this.