I'm reading LE Modesitt Jr's Corean books and an idea occurred to me. In the book the Archon or leader has a homeworld and various alector's or mages if you will have sought out habitable or near habitable worlds and they try to terraform them in order to increase the ecosystem's life force which gives them power and long life. They push this lifeforce into tubes across the vastness of space or between "tables" within their cities on each planet allowing for teleportation for those with the Talent or Essence to do so. Each planet can only support so many "talents" or Sovereign like beings before the biomass fails. After awhile they send scepters of power that anchor their life force and power on each world and eventually they change their homeworld as the old one dies.

So, to make a long story short it would be interesting to have players start on various worlds in a galaxy and vie with each other until one gains mastery of a planet. Then they could study magics that would allow them to find other worlds nearby. They could then attempt to link the worlds and terraform them. A certain amount of resources are lost in translation to other worlds and to make things interesting the Sovereign or one of his heirs can then go through to the other world once ready. Each world could run on slightly different Laws of magic making only some of the basic spells usable without redoing the research.

Each server could be a different galaxy. A second expansion pack might reveal a spell that allows travel between galaxies.

You could have thousands of different races and it could end up that minor factions would be the worlds in which they didn't develop a spell of mastery or learn how to travel off planet which would present a huge challenge to the player but provide an already terraformed planet or even more interesting require your armies to adapt to their world. Imagine trying to adapt life with magic to survive on a gas giant or use Dragons to travel space like in Greg Bear's Songs of Earth and Power or Raymond E Feist's Magician series. Different places would have different amounts of mana and the amount of life in an area could fuel different magics.

An alternative would be to have various alternate dimensions of the same world that just happened to develop differently like Sliders and that after Mastery of your initial world you become aware of the other Shadows a la Roger Zelazny's Chronicles of Amber and try to travel and dominate them running into locals and occasionally other Sovereigns seeking to expand their empire.

Eventually you can smack up against other Sovereigns on worlds that are potentially really viable or another Sovereign could attack and take over one of your worlds while waiting for it to terraform et cetera. It could either go similar to MoM with the green towers I think that switched to Myrror  or you could go the opposite way and be similar to Master of Orion but with magic rather than tech. For some reason an Arthur C Clarke quote seems appropriate:)