Yowza, good luck, that sounds like a huge multi-year game project. My buddy and I finished a mobile game... one that I thought should take 6 months (endless runner) working on it full time. It took 1 year and we are building on top of it now, so, in total we've been building this game/engine for 16 months. We worked on this full time. @_@
I can't even imagine doing an RPG yet. Unity will give you a leg up, but you'll still have to build all of that yourself (or cobble it together from various assets from the Asset Store). Context switching between coding and art is tough. My buddy did all of the art (UI design/art, 3D modeling, animation, iOS screenshots, etc). I worked every day on the code to support everything he wanted/needed.
How many people will work on this game with you?
If I were you, I would figure out the core thing you want the game to be, put all the other features you'd love the game to have into a backlog and focus on building one scoped and focused experience and then expand on it. Playtest and share your game with everyone, take feedback, and try not to spin your wheels (that's a harder than you think).
I don't want to dampen your spirits, but rather temper them. Gamedev is fun but it's hard mental work.