Aurora 4x is a great example. It's a highly niche game developed by a single programmer that models variety of systems and is completely free. It's hard to get

Aurora 4x is a great example. It's a highly niche game developed by a single programmer that models variety of systems and is completely free. It's hard to get into, but the systems it implements compliment each other really well to create a realistic space empire building game. Examples you mentioned above are all "mass market" (except DF) in comparison.