And then edit it.
But then again, specialized game engines can be used to cater towards your needs.
I should look into making one. It doesn't hurt to look things up, except research torrents?
It would certainly be interesting, I'll give you that. But with anything interesting, it will most likely be difficult to pull off.
The concept I got flamed of a year ago. It surely would be hard to pull off but it would be quite epic. Join guilds, make epic flying fortresses, blow the crap out of people's boats and gather up the pieces, tame beasts, conquer houses. I am surprised no one made this yet.