Illusa is a game built specifically for VR headsets, and I was part of the small startup team behind it. My job was gameplay programming and level design, and the piece I handled was flight: the physics calculations and movement simulation that decide how the player actually moves through the world.
flight is a hard thing to get right in VR. In a flat game a floaty or stiff movement model is a style choice. With a headset on, movement that doesn't match what your body expects reads as wrong almost immediately. So the work was less about big features and more about the numbers underneath: acceleration, momentum, how quickly things settle, until it felt believable instead of merely functional.
level design came with the same constraint from the other side. A level has to be built around how flight moves, not around how a walking character would, so spacing, scale and pacing all had to be checked against the movement model rather than a top-down plan.
where the project needed something custom, I built the gameplay system from scratch. That, plus the physics work, is what I took away from Illusa: VR rewards interaction and immersion, and both come down to details you only find by putting the headset on and testing.