Well the only way to share maps currently is through the Marble Marcher discord. Adding a server to store maps was a thing I thought about, but the range of possible levels is a bit limited, so they get pretty boring and repetitive with time. I'm working on a new engine with a better map system(you can add various objects, even triangle meshes), I think after I finish it I should try and make a map server. Though it is pretty far in the future.