I played Subnautica while Sea of Thieves was in its rollout and I found myself wishing for a combination of the two, with Sea of Thieves’ open-ended gameplay, camradery, and station-switching mixed with the verticality, and varying gameplay, and persistant world of Subnautica.
Balancing could be done by having classes w/ skill trees who bring one or more components to a submarine, depending on ship size – ie in a ship of 3 the scientist could bring a mining laser, a tactician could bring a turret, and the pilot could bring a smaller recon sub. Meanwhile a singleplayer could mix skills and ride around in a Seamoth.
At a central hub, a crew could be assembled and a ship loadout arranged. Additionally, each class would be able to use each component differently, which would encourage shuffling stations (a la Lovers in a Dangerous Spacetime) depending on the situation.
People or groups could build and upgrade bases where trading and loadouts can be customized. PvP action could be limited to zones, like in Eve, or altogether, depending on the server, and PvE events vs. big sea monsters would be a good time.
What are your ideas for a single-player game that could be modified into a cool multi-player experience, and what kind of balancing and gameplay changes could help it get there?