Operation Lifeline was a couch co-op game with spaceships, which includes two of the most fascinating things in gamedev. My contributions here were mostly in terms of physics tuning and lighting.
When different ideas got bounced around in our chats, I made a "workshop" scene that randomly made levels based on several controls so that the team could test out various design options, such as how physics should work, how quickly things should happen, how the controls should respond, how large the levels should be, etc., Every time someone had a new design idea in the chat, I updated the workshop.
I also helped with setting up the levels, the transitions between scenes, and fleshing out the game's story.