What kind of a multiplayer do you mean? One verus one? Or are you thinking about creating a whole solar system and everybody can just fly around exploring?

I would start with a simple multiplayer dogfight where you start off face to face and you battle it out in free space. Then you could add fleets that back you up. I think that would already be a nice game by itself.
But if you want to take all this to multiplayer or expanding it to a whole galaxy, that would take a huge amount of time do develope.
Eve Online wasn't built in a day

so long...