Something like that. Talking only about single player and making it immersive, I’m sure for multiplayer it would create many unnecessary problems that are far beyond just adding two boats and a button.

1. Turn the “Copy Character” menu button into a 3D asset on the map you interact with. Same button, but it’s on the map (in shape of an interactive boat in a harbour)
2. Instead of “Copy Character” it says “Travel” for example
3. Then you load up the other save / map as you usually would and it puts you at the harbour with copied character as if you traveled.

Basically all as it is now, but with two 3D assets and placing you at the harbour when you load up the target map.

*The mechanics of single player “Character Copy” function wouldn’t need to be changed, just a make-believe cosmetic that makes it more fun to roleplay.