I think he ask for Online game, not Multiplayer massive online game.
I mean, I wont voted for MMO base on X3. I hate MMO, and seems all nice games are going MMO lattelly , which is a pitty(KOTOR, Warcarft, etc..)
BUUUUUUUUUUUUT think about Animal crossing

.....really, think about that.
Lets say, I have my universe, my X3:R or TC universe. And there is one place, where I can find a wormhole, If I use that wormhole, I can go to another dimensions and visit another X3 universes. For example I can go with my fleet to the X3 universe of Blake Gedye, I can interact with him, figth with him, help him on missions, kill him

, and so many other things. Of course, he has to invite me in order for me to enter his game.
Anymal crossing is base on it, each player has his own twon, and you can visit others, which are same rtown but with different things. Let say you blew out all Xenon from your game, I will still have them.
Is just one example of an easy way to make Multiplayer game, with no need for MMO...