Don't try to do too much with this mode. That doesn't mean don't put effort into it, but I have seen requests for:
Online 64+ player persistent worlds. This isn't NWN, and it shouldn't be. Competitive GMing, with switching GMs (Sword Cost Legends Style) GMs that can magically make major edits on the fly, including building a module while players are playing through it. Ability to GM the main game, including rewriting it (also on the fly?)
Obviously, most of these are incompatible, and questionable even by themselves. What I would like to see:
Multiple levels of GMing, based on the map played. 'Locked' maps (such as the main campaign) might allow limited GMing, such as controlling enemies, adding / changing traps, and just generally tweaking the difficulty of the experience. Levels specifically designed for GMing would allow many more features.
GMs should be limited to a reasonable number of players. The standard would be up to 5 (4 players and 1 GM), but if the game could support a few more (no reason it can't, if it seems to be able to do up to 5 when 4 is standard), then up to 9 would be nice.
GM should be able to: Add/remove monters/traps Control monsters Add buffs/debuffs to parties/monsters Edit monsters (add abilities, stats, levels, XP, drop table, etc) Add remove path blockers (eg a wall that dissapears when quest progress is made). Create on the fly dialogue with NPCs. Place loot
Ideally, this could all be done in realtime while in game, although probably some work would have to be done beforehand in the editor. Actually creating maps, adding areas, etc should all be done in editor, not in game.