A bunch of stuff see description
To start with, some basic element physics such as flowing water, sand needs support etc. Should not be too hard there are some decent elements already build in to unity. Next would be letting the game load texture's externaly (for standalone) so modded texture packs are possible. More admin/mod functions, more options for proctecting builded things against the well known griefers. For any question feel free to mail got a lot of experience with unity, using A.I and Advanced physics.