I'm easy, and my only main requirement is that the user interface be every bit as efficient as MooII, playable by track ball or mouse, and include a tabular, sortable, summary where the player can complete purchases from savings (if global money is available) or be re-routed to the appropriate game screen - etc, etc, etc. Also, user configurable game options are a must so that users and tweak variables to thier own tastes (spying as a selectable game feature, for example, not as a core game function). And... Keep ship movement structure and turn protocall similar to MooII, not as in GC.
Lastly: Efficient Assembly Level Programming that screams on newer PCs and runs graphics in the hardware mode, even if just 2D! Let's not move backwards in high speed execution of game programs done with nothing but simple graphics! I don't want to see any slow moving ships or slow game fuctions while using a typical modern high speed PC.
As for content, MooII and GC provide a lot of truely unique and original concepts worth retaining; as I'm sure can be said for other 4X Space games as well. Reinventing what is already successful and already invented as proven 4x protocalls and concepts typically leads to failure and subsequent waste developement funds --- rather, build upon already successfull concepts and ideas where the fan base already waits eagerly for such. Add new concepts, ideas, and game structure protocalls cautiously. 3D enviroments are not necessarily a priority if fast program exectution will suffer.
The developement team should already have a firm grasp 4x game play, game mechanics and dynamics from the more sucessful 4X games, including, not limited to: MooII, Stars, Reach For The Stars (it started the 4X story in the retail markets), GC/AP, Spaceward Ho, Pax Imperia... I probably forgot to mention some others as well.
Keep some humor in the game...
I'm a single player fan and like random situations the best (user configurable). The multiplayer on-line aspect is my least concern.
Good Luck and Best Success to you and your team.