Neverwinter Nights style with persistent world, and DM Client who can make quests, will be perfect.
Tools for moding, DM Client (give xp, make items, put monsters, npc, roleplay with this npc, make quest for players, make worlds alive), and support for players servers with persistent worlds (no original story, just stuff make with modding tools).
This will be perfect game... Not so hard to make. Obsidian, just think about this, and don't close this door. Modding scene will make all other work.
With this option you can also play with friends in original campaign, but can make much much more.
Pen and paper sessions in game.