Open world game design poses some quite significant problems. With a corridor shooter your environments are narrow and tightly focused, so you can stage content more easily and don’t need so much of it.
With an open world the challenge is to fill it with enough content (quests, dungeons, enemies etc.) to avoid having loads of empty, useless space. That’s going to be a LOT of unique content even though you will likely reuse assets as much as possible (cave A is just a mirrored version of cave B for example). Don’t forget also the challenge of reusing content without the player realising that this dungeon is just like that other dungeon they explored a few days ago. This is why many games use procedurally generated dungeons to ensure randomness, but even in games like Diablo 3 it’s pretty obvious the computer is just clicking prefabs together.
You will also need to take into consideration the fact that the player is more free to tackle content in whatever order they choose, so think about the difficulty curve and how you ensure players stay challenged as they progress without having too many walls blocking off content that’s too advanced.
In all, an open world RPG is a serious amount of work, even if your world is fairly modest compared to, say, Skyrim or Witcher 3.
Good luck anyway, but be prepared for a mountain of work!