The only way I could see multiplayer being added was if a lot more indirect than the more traditional multiplayer. A good example of the this would be Subnautica's "Time Capsule" mechanic. A quick run down of what the Time Capsule in Subnautica was incase you don't know, when you complete the game, you are given the option to leave a Time Capsule for another player to find somewhere in the world. The Time Capsule would be filled with a few items, along with a message, and a picture to add a bit