Here are my thoughts on migration. It should be removed as a game mechanic.
Instead, buff the alliance system so that allies can loot each other's resource nodes and dungeons.
The amount of alliances allowed is determined by the number of cities held by a kingdom. If the kingdom has 1/3 or half of the cities, they get no alliances. If a kingdom is small, they get the max number of alliances.
People in losing kingdoms still get to gather resources and fight, people in winning kingdoms still get to be dominant and control a majority of the map.
Besides this, something has to be done about super-whales. Because one person shouldn't be able to wipe a server. Fix those things, and I think this game can last a long time.