I don't know if this has been said yet, but I would love:
- the ability to design branching dialogue trees
- hidden quests (aka there would be no quest marker over specific side quests if the dm wanted but if a player was curious enough to click on it would see the quest and could accept)

Other things that I think would be cool are:
- custom Items
- Friendly monsters
- Recruitable NPC's
- (this is very out there but) the ability to give recruitable NPC's dialogue in certain situations.
- friendly recruitable monster NPC's (it's just an Illithid, don't worry about it)