I see a relatively simple solution: Each federation member must agree to the change before a new member can be added to the federation. That way, it gets increasingly harder to grow federations, because you have to get more realms to agree to it.
In fact, I assumed this was how it worked until very recently.
With the way it's currently implemented, growing a federation actually gets easier as it grows, as more members = more rulers = more likelihood that someone will decide to unilaterally invite another member. And no one dares to leave the federation, because how do you know that this is not some conspiracy to get your realm to leave, and then everyone gangs up on you?
Of all the malicious pranks pulled in BM, I'm surprised that there has not been an island-wide federation blob yet.
