Sounds brilliant! A bit like Master of Orion or other 4x game but with more emphasis on economy and trade, I like the idea of an open market.

MOO, allows for different ship designs so the user can design there attack, bomber etc... perhaps this could be done by only certain planets? So you have to buy your ships again on the market.. MOO also has a tech tree so different players come up wth different tech although this would be a bit unfair joining the game later than another player but the tech tree could be controlled maybe by yourself?

Different races in MOO have different perks, like the one species gets a planet assault bonus, another is better at production.

Sorry am going off on a tangent but hopefully some extra ideas