I'm currently making a Neverwinter Nights module intended for roleplaying multiplay. I've added what I think would be needed in this - extensive DM support, checking the legality of characters, respawning treasure and monsters. Are there any other essential features I should add?

Well, one of the most important features of roleplaying modules is Character respawn. If there is any of course.
Like you can freely respawn and lose 10%xp and gold, or impossible to respawn and can only be ressurected if someone else finds your body, is willing to use his divine powers on you and if you have enough cash to pay him up (ALFA ;) ).

Thats what i can think of now.

Ah yes, I'd left that out of my list. People can pick up your corpse and take it to a healer, or you can choose to respawn, but lose a player level.