A prototype for the planned gameplay. Just basic movement and particle setup.
We've never before worked on such a project on our own and have quite different experience with modeling/programing/designing, so it's very important that the fundamental elements of the game are not just defined in some game design doc but that there is an actual prototype early on, so that we can experience first-hand what others imagine it to be like and avoid 'but I thought with X you'd mean Y' moments as early as possible.