This scale will take you many many years. Don't forget, that a large world also requires to be filled with content, a task heavily underrated. Skyrim had dozens of people creating content, but it's still empty and boring. Now imagine you have to do this yourself. If you are doing it alone there's a high chance the huge amount of work (altogether) eventually brings you down, causing you to lose interest and then all the work was useless.
Btw. I just recall another level of abstraction. When I first heard of Might & Magic X, rumors were about "doll-house" models. I interpreted it like this, that you have some kind of miniature representation of towns (same as a base in Heroes of MM), 1 single model representing a whole town. Once you reached the dollhouse you'd see a 2D townpicture with hotspots. Now this sounded totally cheap, and it didn't turn out this way, but I would have bought such a game just for the sake of seeing this level of abstraction.
Investigating this, I'd consider the following abstractions as doable:
- Doll-house buildings with 2D representation on the inside (like in HOMM)
- Enemies on map as moving placeholders (like in Wizardry: Tale of Forsaken Land)
- Combat represented in a first person screen (like in Lords of Xulima)
Basically it would similar to HOMM5, but from 1st person view. Hence the abstraction would also allow Cooperative play like in Heroes 5. You'd also save a lot of time in pathfinding, as you don't need group AI or avoidance AI.
But even if using THIS level of abstraction, it's still a lot of work. Think of the skill system, items to create, enemies, balancing, spell visualization, sound effects, music, story, 2D town screens, miniature models, designing the landscape so it's attractive (the HOMM series somehow achieved a magical look with all their glitter, bling-bling, although the graphics weren't that great).