I think there must be the freedom to do what you want as there is in single player. Maybe having a seperate multiplayer and singleplayer universe would work well.
-The multi universe should be small, the point being you play with your friends.
-It could work quite well in a similar style to some RTS's where multiplayer games are saveable so you can resume them later (Empire Earth comes to mind)
-To resume the game you should have most players present (perhaps 2/3rds).
-Any player that is not present has all stations controlled by the computer, ships that are running on auto-pilot continue. Any ship that the player was flying or does not have orders is temporarily removed from the game.
-Property of absent player still in the game should either be invulnerable to other players but destroyable by the computer but some compensation paid for anything destroyed or perhaps completely invulnerable.
-Each player is a faction in order to set Friend or Foe settings.
-There should be several "wilderness" reigons made of several empty (+ roids) sectors for player expansion.
-These wilderness reigons allow for players to either build their own individual empires, 1 empire together, or several opposing empires in teams.
-If players ally to create an empire they count as 1 faction and are able to declare war or make alliances etc. with all NPC factions. (Will require some sort of diplomacy screen, or even better diplomats and embassies)
-Start conditions should include things like starting money. This means you can all start from scratch or all start with enough to build your empires and slog it out, or anything in between.
-There will also need to be sort of team work systems, such as the ability to form squadrons/gangs/fleets/whatever else you want to call them. When one member of a squad accepts a BBS then all squad members have accepted it. Prize money is split evenly between all players.
-No PvP should be an option when starting a game but should not be adjustable during the game, after all, thats what you all agreed to at the start.
-There should be some sort of script limitation, i.e. no scripting objects in/out by players. Scripts, such as software or mission scripts, should still be allowed so you can add extras into your game.
-Another starting option could be race. This will determine your race rankings at the start of the game. All players start neutral to each other. And possible an option to be neutral to everyone.
-I think it would be good to have the ability to let somebody else control your assets if your not there, be this in game by handing over possesion temporarily or physically, as in let someone else play your game (don't know how to do this though, possibly just a simple save game transfer).
-Allowing new player to join would be good as well only by agreement of all players, perhaps through some sort of voting system. New players should start with the same conditons as all original players did.
-There was more but i've forgotten it. (but i think i've remembered most of it now)
-And this is by far the longest post i've ever written on a fourm....
] in time to hear crappy voice saying, 'That'll teach him' and then lights out. Always tended to be the worst spec player that caused the lag. Usually a modem player! 
