All I really want on top of what I've seen in the GM-mode videos is "automated area transitions" and the ability to place traps. Especially the automated transitions are important to me as I would like the players to explore outdoor areas and enter caves, dungeons or go through trapdoors without me having to teleport them.
Number 1 on my list is to be able to transport characters from one campaign to the next, or the ability to keep on adding newly built areas (including those transitions) in prepare mode for ever to an existing save.
