In my opinion, based on the number of my psycho sports gaming friends who have not stuck at this game (but will play Strat-O-Matic or fantasy until and after they get divorced for it) this is a niche game, and demand won't radically increase with advertising.
If I'm right, the steady state for this game with the current number of worlds is probably at a couple hundred or so empty slots, and a number of very slow rollovers. I see only 3 ways to deal with delayed rollovers-- sims, adjustable size worlds, and merger/contraction. In a perfect world you could do all three.
I can see sims being non-terrible as long as sims can't trade. Adjustable size worlds are probably hard to program but I would think they would work better than mergers. 3 into 2 and 4 into 3 mergers are probably better than what we have now, but I suspect that there would be more attrition with that option than with the other two.
The key, as I see it, is that private worlds could have any or all of these options available to them. Stuck at 25 for 2 weeks? A private world could find a volunteer to leave and roll as a 24-team league, or add 7 sims and roll today, or just wait it out. Public worlds would probably have to have hard and fast rules, something like, "After 4 weeks, if you have 25-31 owners you fill with sims, if you have 21-24 owners you become a 24-team league, and if you have 20 you sit around waiting to merge with another world." You'd also want to cap the number of sims at around 7, if for no other reason than a commish could scare off the other 31 owners and make a private world in which he's the only human into a profit machine.
All this is easier said than done; the programming for mergers seems somewhat labor intensive, and a decent sim or floating world size would be even harder. But I'd like to see one or all of these made available.