This is a big one to ask for, but I would love for Civil 3D to allow for hydrology and hydraulic calculations.

It would need to accept all typical hydro. information: watershed area, runoff coif, time of concentration, starting HGL, etc., and also have a pipe routine to calculate HGL's automatically. I would like to pipe routine to be linked to the pipes in the profile for seamless generation of revised profiles. I would be surprised if Autodesk is not already working on this.