1. Allow a player to create a multi-player game and invite a friend to join your game via TCPIP for PC and MS or Sony networks for consoles.
2 Mods (on PC) will be disabled in a multi-player game, but DLCs will of course be supported. (To avoid players having to match up their mods in order to play)
3. Allow the other players to control your follower. Followers can pick items and kill stuff, but followers cannot advance quests.
4. You can bring up to 3 followers be it AI controlled or player controlled. The game scales slightly(with each additional follower you have.)
This is the best way to implement coop without having to deal with hacked characters, mod incompatibility, different quest progress, duplicated items etc etc because the helper players bring nothing from their game over to the host. Everything comes from and stays in the host game.
When the helper players quit or join, the host can continue his quest without even pausing, and saving or loading the game will not result in sync problems.
In other words, it plays just as single player game except your buddies can occasionally jump in to experience the game with you. Zero comprise on the single player experience.