SpaceRoad is another solo Unity game, built around a gravity concept that turned out to be genuinely fun to explore. The whole game is the mechanic: bending the pull of gravity to navigate, and discovering what that single rule lets you do once you start playing with it.
Like KIKA, I made all of it myself — design, levels, and the C# that holds it together. The interesting engineering was the physics: getting real-time gravity to feel responsive instead of floaty, and tuning it until the core loop was satisfying on its own, before any content sat on top of it.