Move all bases that have been inactive for 30 or 60 days to a limbo realm. if they become active again they can request to be transfered to their original realm or a new realm. This would reduce the need to create new realms and can better merge two realms that are low pop together, before making new ones. Any bases in limbo will not be able to be hired as allies.
Thanks for the feedback! Generally cities are removed from the realm after inactivity, but this depends on their account level. Removing them from the ally list would likely cause issues with players that already have them hired losing a potentially valuable ally, so I'm not sure how viable that is. Nonetheless I'm sending this along