The empty paintable map would turn to a paintable map via right clicking. Then the player would be able to enter a name to the map. The maps wouldn't be stored per ID, but per name.
The paintable map wouldn't render the terrain automatically to the map. A waypoint would be created via right clicking with the map in hand. Then the player would be able to give the waypoint a title. It would be optional, because there would be no name required in most cases. The waypoint would render like a red cross with a title (when entered) on the map.
Any dyes can be used, but they must be different colors.
Mapping table would be used for painting into a paintable map. There would be a slot for pecil, a slot for painting tools and a slot for paintable map. The pencils and painting tools wouldn't be stackable and would have durability 160. You would see the waypoints on the map and you would be able to draw here anything you want. The map would have resultion 1024×1024 and color depth 8 bites. There would be two tools for modifying the picture. Paintbrush and pencil. You would be able to set the color and width of the paintbrush. Using this tool would damage the painting tools. You would be able to write labels via the pencil just by typing the letters on the keyboard. You would be able to set the text height. The labels would merge with the picture layer, so just use paintbrush for deleting the label.
You would be able to move with the whole map to any side, scale it up and down infinitely, but every time, you would do this, it would clear the picture layer. The mapping table would show the size of terrain, that is shown on the map and the position of the center. There would be an option, that would tell, whetever the player's position should be shown on the map. The map could be locked via a button, that would be also in the mapping table's gui. Locking a map would remove all its waypoints.
The locked paintable maps could be copied like normal maps do.
And remember that, the normal maps would still be here. Normal maps couldn't be edited via the mapping table. You would be able to choose, what would you like.
You would be able to paint own pictures. (Just don't use the waypoints and hide the position.)
Mapping via those maps wouldn't require the terrain loaded, that is good on servers with limited view distance.
You would be able to create much larger maps much easier than before.
Passing whole the terrain would be no longer required for creating a full map. You would just have to know the terrain and take some waypoints.
You would be able to put an image of your house, how it looks from sideways, to your map.
You would be able to put labels into your maps, so you would never get lost.
You would be able to create a map of a cavern or a map of your train system.
There would be a possibility to highlight some places by putting some marks here.
Painting a map could last a long time.
You probably wouldn't draw the terrain so exactly, as it really would be. Normal maps would be still here.
The possible variant of this suggestion is getting rid of paintable maps and adding these features to normal maps. They would no longer have these two disadvantages. However, this wouldn't be so compatible with the current maps, so it's not the main part of the suggestion.
Finally some respones, thank you.
And also thank you for support!
Those maps with own content would be better than the maps created via external NBT editing in at least two ways:
They would be available also in survival mode without cheating.
You could know your position on the own-painted maps. It's impossible with the NBT-created maps, because they are updating, when the player's arrow is shown on the map.