Domination of worlds by alliances and players who have already defeated the fortress.

That is a nice idea. Famous last words here, it probably won't take much to produce either. They've already got multiple hubs sorted on Veteran worlds, and the addition of one or more fortresses would be a logical development of those worlds. Replace a few of the hubs in each sector with fortresses, link the fortresses to their nearby hubs in some way (colour maybe) and you have your idea implemented.

Any more sensible suggestions?