Portable Characters that have their own save. Similar to how the game Grim Dawn works.

[Rabbitslovecactus]

Your character is portable, you can go into any SP and MP game with it. Games have their own saves, and your character(s) also have their own save. This will allow you to take your SP character and easily join your friends in a MP game with the same character. A good example of this being used is in the game Grim Dawn. They even have a trader that you can store your loot with and then join the same gave with another player and have access to the same loot. It's a very flexible system.