I am already skeptical that the GM mode won't become another day/night cycle which gets cut for being a lot more work than anticipated. But I am really skeptical about the idea of being able to "construct levels on the fly".
Even if it's possible, I'm not sure that would be fun to play. D:OS levels and rooms have lots of details and items placed. After you add your new empty room, what will the players be doing while the GM is spending 5-10 minutes adding in all the stuff to it? (Per room!)
This feature is important for persistent servers. The world is already created but a DM must be able to make little temporary modification without needing to modify the map in the editor and without restarting the server. You must also consider that players RP between them, XP in automatic dungeon, and do their stuff even if DM are not connected. You are right for "campaign mode" where players are always with the DM.
Per example: Bad guys attacked an house and now stay in. The DM spawn some bad guys in the house when no player are here, moves the placeable to illustrate the fight in the house and then spawn a guy in the city looking for player help. You can do the same by modifying generic maps (inaccessible by the player without DM help) for a specific animation and then creating a portal to this map.
In persistent module, not all DM are allowed to modify the module, but only to create animations for the players on the fly. Only permanent modification to the world are made through the off-line editor.