Heya!
I was wondering if you would consider adding Flood-fill as an option for modifying maps. I feel this would work really well for converting outdoor maps into underground maps, and vice versa. If bounded by the edge of walls would create really quick and easy way to edit a large map. This is especially useful for Donjin and Single Page Dungeon auto-generated maps.
Below I've done an example by hand. Where "flood fill" removing the wall instantly converts the map from underground to an above ground dungeon. And then the second example which quickly fills in the outside floor terrain.