I'd like when making a map to be able to mark locations. Making a marker by have requires big structures to be built on the top level of the map. What I was thinking is there could be a block that you can put anywhere, even underground and when the map updates that area will drawn a picture like it does with the woodland mansions.
Here's the idea. A block that is constructed that holds 1 non-block item which will be analyzed to determine what the marker will display. This block can then be planted underground and be out of sight. So what icons would we see?
Interesting idea... Although, would it display whatever item is in it on the map, or just a different colored dot? That's my one bit of confusion. Also, could you put a crafting recipe for it up on the Original Post? That would be much helpful.
The basic idea isn't that great, because it's already in the game. If you make a map and then make copies of that map and place the copies in item frames at various locations, they will show up as markers on the map. I used this to mark villages on a zoomed map. It does seem to have problems if you have more than three markers on the map, though.
The only part of your idea that is different is using the different shaped markers that are used in explorer maps for woodland mansions and ocean monuments. Not sure if we really need an extra block or item, just for that one feature. Might be interesting to rename an item frame so that any map placed in the frame will change its marker based on the name of the frame.