It would take a ridiculous amount of effort and be used by almost no one. It doesn't make any sense, and I for one would prefer that they spent their resources making more game. The D:OS2 DM mode hardly got any use.
For a DM mode to be sensible, the game has to be made with it as a primary and core functionality in mind from the start. Take a look at NWN, they made the Aurora engine more than they made the campaign. The campaign was almost just a barebones example of what the aurora engine can do. And it sold like crazy, and the number of modules created is staggering and still going strong to this day. This is not that kind of game, and I daresay we won't see one like NWN created ever again.
