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.
I think i'd make a cute side-scrolling game. Mr. Lily has to save his seedling from the Beezies every level. He has the Nectarlings on his head, they can stick to walls and climb... they also fight off the Beezies with goo. Mr. Lily can "root down" and grab objects under the dirt like rocks, and toss them at the Beezies.