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 ..