Designing "Topdown"

I've begun the design work on a topdown, 2D, exploration based game. For the time being I'm just referring to it as "Topdown" until it's more fleshed out. I'm not going to break ground on development until the game is entirely designed. It will have RPG elements and be set in a fictional ancient universe. I want to ensure that the backstory, and main story line if it pertains, are simply setting a space for players to create their own story and experience. The world will have a theme that's mostly realistic with some fantasy elements and a simplistic visual aesthetic. The world will generally be dark and gritty in a very serious sense, but at times it may give way to clean, safe areas to break up the flow. I'm compiling a list of tasks that need to be completed for me to consider the design "finished". I've got a ton of ideas I'd like to implement, but chances are I'll be doing a good bit of pruning.