I too think it's a good idea. I once built an open world game similar to yours, except in space. Personally, I had no idea about game design or construct and wasn't able to create anything like what I wanted.
Three months later, I came back to my project. Instead of taking it head on, I built a smaller game which had the combat mechanics, another one that simulated a universe as AI moved about and interacted with each other and countless others. I combined them together, and I had a fully functional game that actually worked well.