I'd be a 3D platformer and online co-op. It would focus on communication and coordination to solve puzzles in the levels. Example: One player could anchor themselves to the ground while the other player spins around, then disconnecting the tether thus flinging themselves across a gap to push a button. You could help your teammate wrap around enemies to defeat them (pictured). Perhaps pull each other back and slingshot off of pillars like Angry Birds. The ideas are endless.