I don't know if this has been said yet, but I would love: - the ability to design branching dialogue trees and - 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)