What if all killed bosses will be marked on the map? Like with some X on them?

would love to have this. and to take it a step further, you could enable/disable automatic X's or some other icon appearing on map after u beat a boss. and u can select to add or remove X's to areas too. So like if you wanted to cross off a hollow hill after looting it, even tho theres no boss, you have the option to do so yourself. And make sure this carries over when you make a copy of the map.

Otherwise the current idea is to go over with banners and I think theres a way in one of the recent vanilla updates to put icons on the map based on where a banner is. but obviously this is would be super annoying to go back there

This isn't possible at the moment without reading world data which is very expensive and will drop tps

Maybe instead of trying to do it automatically if that's too expensive, there can just be a manual option to add a marker over certain monuments. So the player can just decide to add/remove a marker whenever they want.
Never thought to ask this idea but it would help me so much lol my current way of keeping track of whats already explored is such a mess