I found a forum under Off-topic for a game called Astra Terra. It looks like it has some floating islands in it (like minecraft and Terraria). I wonder if Red would consider adding this to Rising World as well or if he is looking for a more natural world. If floating islands could be reached, perhaps they could be mined for an unobtanium-like ore that levitates. The stone itself could have a neutral gravity setting (I'm assuming this is how tops of mountains stay up in Rising World after you mine below them). Perhaps this could be a crafting item for fantasy-themed vehicles that float (like airships in Final Fantasy). A system would have to we worked out to manipulate the gravity value on a vessel in order to make it fly.
I realize Rising World has more of a realism appearance as far as the items so perhaps magical devices (like in Minecraft) are out of scope with what Red envisions in this game. It's just a thought.