Echoes of the Eye expansion to Outer Wilds. I managed to avoid all the spoilers, watched some playthroughs but thankfully didn't study them too closely. Importantly, the streamers never looked "up" during the parts of the gameplay that I've seen, so to me it appeared just like another normal environment (well, normal at least by Outer Wilds standards). I already loved the original game, and decided I must play this for myself.
So when I entered through that doorway for the first time I was genuinely stunned. "You fuckers, you really did it this time. You actually went ahead and did it!" I mean...
spoiler
Space habitats have always been a staple of science fiction novels, and they have appeared a couple times in video games already, like in Mass Effect and Halo, but there they were only used as background - the actual playable area was limited. Never before this had anyone successfully implemented a life-size Bishop Ring with the full "You see that mountain? You can walk there!" boastfulness. And sometimes that mountain is on the ceiling. And when the water breaks, oh boy...