well first off, how big are you planning to make it?
is it going to be turn based or interactive combos? [if you plan on making this multiplayer 2nd option will be limited]
any plans of a mastery tree or skills/passives being used/acquired?
any type of plot? [personally I'd recommend something simple if you do]
any type of NPC's? [would make more sense if this was more guided toward single-player]
way of engaging combat? would it be more freestyle, or would there be conditions restricting the challenger or opposing party from accepting or denying engagement
how's the inventory system going to work is it going to be all in one place or separate places like chests
^^ this will lead to a crafting system along with many other items, will there be recovery items as well?
^^ how dependable will the resource farming will be and possible cooldowns