No problem - both Unity (C#) and Godot (GDScript) can do 3D, though Unity is probably more common for that, also keep in mind that open world and MMO are 2 of the most challenging genres in the industry to make, even for AAA studios, so you could be looking at a multi-year development, and with multiplayer/online there is often a financial cost for servers - if all those sound like non-problems, then im sure youll learn a lot from the experience