Destination Sol: 2D space arcade/RPG with seamless open world & planets. Source: about 2 years ago

Extra large planet with extra wide atmosphere that causes your ship to fly with aircraft like physics when you dip into it. There are cities, gun turrets / missile bases, tanks and/or creatures running around on the planet's surface. Some will shoot at your ship(s) and your goal is to destroy specific targets among these on the surface. The gravity is fairly strong when you get close to the planet, so you have to... Source: almost 3 years ago

There is also Terasology - a Minecraft-like voxel game with fancy graphics: https://terasology.org/. - Source: Hacker News / about 1 year ago

You might be interested in looking into a few Minecraft clones if price is a concern - Minetest is the most well known one from what I've heard but I was recommended Terasology a few years back. Source: over 1 year ago

Terasology has active contributions, but the link to it is super old. terasology.org should be the current. I think the GitHub link is correct, though. Source: over 2 years ago

Terasology would love for this stuff to be easier, but is unlikely to use anything closed-source for a piece as integral as the application updater. Source: about 3 years ago