A sandbox campaign would suck. If you pay attention, the campaign has many of the same missions across the factions but are tweeked for flavor etc to the point you don't notice it. that said a Ork campaign strikes me as an obvious next contender if they do another campaign. or Eldar. Tau is right out as they're not active in the eye of terror region.
those all strike me as important things, especially as the campaign progresses, not being able to move from the old battered Luna cruiser to say... an emperor class could be a bit of a problem. this is doubly so in missions that require you to keep so and so alive.