Then the maintenance thing is basically just an ISK sink, saving some of the logistics headaches for those operating in null-sec. Instead of having to get Ice products to the towers, you're just paying the maintenance fees...
One thing they can do, to put a cap on the size of the various domains out there is to make the fees use the same sort of formula as the skill training. Say the number of constellations is the same in the formula as the rank of the skill, with each system costing the same "overhead" based on the calculation.
More complicated options would be a cost per gate, but again with the per constellation modifier I would think.
Or a sliding scale that generates per constellation per star. (As above, but each star system is equivalent to a level of training at that grade.) So one star in one constellation, and you're looking at a fairly small cost. Six or eight stars in one constellation and now you're talking serious cash. Add another constellation and you're expenses just went up quite a bit, even with just one additional star in the new constellation.
But even this sort of situation could be gamed. If it's constellations=rank, and stars=level, you could run an enterprise where just one or two stars per constellation, running across three or four constellations, might actually be cheaper than taking a whole constellation itself. (Same number of stars total.)
Might be best to have constellations=rank, and sovereignty level (or it's new equivalent) be the level of cost. Familiar equations that make things more expensive per level and rank...
I'm also interested in finding out if they will apply the same structures to W space. I know it's not supposed to be "colonized", but I also know the players frequently do things the devs don't want.