I am not sure that a decision to arbitrarily close any island would be received all that well. Even the players who recognize that it could eventually be a good thing for the game are unlikely to able to take it without a fight. I can't even imagine the resulting fight over which island gets closed.
I'm sorry, but the only way that I see it happening without an uproar from the affected players is by somehow allowing the players on the candidate islands to fight it out IC/IG for the survival of their island. Telling people "We're closing Kepler Island, you have 30 days to leave" will piss people off, and you'll probably lose a bunch. Giving them some way to fight to save their island is much more likely to be well-received, and less likely to cause players to ragequit.
Can anyone think of some way to do that *without* some kind of invasion?
Well then you'd have to make some sort of "victory condition". Perhaps like in Age of Empires if your realm builds a wonder and holds it for a month you win.
