These are core positions.
Captain
XO
Chief Science Officer
Chief of Security
Chief Engineer
Chief Medical Officer


Swing Positions
Chief of Nero-sciences (CNS/Counselor)
Chief Tactical Officer
Chief Intelligence Officer
Chief Operations Officer

Just a thought. - I'd be interested in doing this as well. If it's based around a single ship, something like this could support up to 24 players and a single GM, or two. Though the CO and/or XO can suffice in this capacity as well. With each department having three junior officers (ensign grade) the total could come to twenty-four (CNS usually doesn't have more than one, but it could). And of course, Nameless Ensigns, Nameless Crewmen are always available for redshirts. - It would be a pretty well defined command structure, but it could easily work as a large RP or small RP. 

Are you planning for this RP to run for just a few weeks, or do you want it to run for a long period of time?
Are you going to be using sources like the TNG and DS9 Technical Manual, Star Trek Encyclopedia, and/or Memory Alpha? 
What kind of posting rates are you looking for? 
Do you have a ship class/design in mind, or would you be winging it?

Sorry if this seems a little in-depth. I come from [url=http://www.star-fleet.com/webb/]Star-Fleet.com[/url] and this is in part how our structure is set up. Course I love the Guild too. XD