I have a suggestion.
Custom bosses that spawn at greifed/abandoned player countries (that are at least a month old).
The idea is that a boss named Shadow of <leader> that has all black dyed leather armor and a full black player head spawn at a greifed/abandoned country every hour. If there were multiple, just make the most important ones a boss. They have as much health as 2 withers, and have different strength depending on how successful their countries were.
feel free to criticize this as much as you want, the only custom mob I made was a iron golem with a lot of health and a bossbar, so i don’t have much experience. also, you probably should add the old ones first. (cliffania, exodus, piarsis, etc)
literally just a copy paste of my old suggestion, also I might add they drop one piece (random) of a new armor called <countryname> armor. that armor allows you to have different effects (and prot 10) depending on where you got it.