Map markers? I don't see that working out.
The maps are largely generative and the area's contents change every day. Whatever you marked won't be there when it changes the next day, defeating the purpose. If you mean to mark dungeon locations, the map kind of already does that for you. The cloudy areas are where dungeons are, and their entrances never change, though the surrounding area might. So I'm not sure how useful this idea would be in practice. Unless, I'm missing something.
Maybe dev could make visiting dungeons remove the clouds on the map and tell you the name of the dungeon when players hover the mouse over it? No point keeping it hidden once you've seen the place.