Limit the walkable area
Ramon Llull
This is one of the most obvious set backs for immersion. How about an option for limiting the area that can be walked on, like lakes, holes, towers, edges.
I would suggest doing that by having the option of uploading another map that only has a white and black area where you can mark the no go areas as black. Another more advanced option would be to have 4 (or combinations of the 4) colors which would imply this area can only be accessed form one of the four directions like a hill with a cliff ..