First thing I'd do is design the data definition layer. This is where you will define your world and RPG system.
Then you build an engine to interpret and animate a world from the data definitions. Unity or Godot might be a good place to start.
Why? So you can change the game engine driving your world without having to recreate your world. This means that when you eventually have to change your underlying technology, the cost of doing so will be low. There are vast numbers of games out there that are essentially dead because they are welded to obsolete gaming platforms (like RPG maker).
Then start small - maybe a small valley, a couple of character classes, a couple of monsters and a couple of quests. Define it int he data layer, then implement enough of the game engine to get it working.
Then release it as version 1. Hopefully you'll get some feedbak and, if you've published the data definition layer, some folks might start making their own worlds.
iterate, adding support for more new things and making the game world bigger.
At some point you'll need to add support for running over multiple federated servers (because the world will get to big to have everyone in the same server), so keep that in mind.