Brief, but enough as a proof of concept. I don't know how experienced you are with game development, but i'd suggest you to make smaller iterations.
Things like composing a soundtrack or developing a rpg system, if you're doing them from scratch won't be trivial, so maybe focus on smaller parts of the game as you can do them. Just try not to make it so big that it becomes frustrating for you to develop it. It's a very common mistake.
Anyway, good luck with it.