COMPLETELY removed from multiplayer.
*Multipler is the exact same as it is right now. Characters are stored on their servers.
*Singleplayer is how D2 used to be. Characters on the computer.
Have it so multiplayer characters can be moved to singleplayer, but singleplayer characters cannot be moved to multiplayer.
So basically like it was for Diablo 2.
Except that having the critical data for Diablo 2 available so the single player game worked the same as online made it that much easier for hacks and bots to be developed for the content, since they could be built and tested in a way that ensured they could not be countered until they showed up online in a ready and working state.
What you in particular are asking for is exactly the way D2 works and the online security for Diablo 2 was terrible thanks in part (but not entirely) to all the critical data having to be client side so offline single player worked.