I think there should be a fourth mode just called Exploration. In this mode you can select a part of the map and just walk around freely as if you were in Adventurer mode. However, you won't need to worry about hunger/thirst, night time darkness, or being attacked. Also everything will be visible at all times, regardless of whether or not your "character" has seen it. The fast travel screen will also work differently. Rather than just "walk" around on a larger scale, it will actually show a map and let you teleport anywhere. You can also select from a list of sites and structures to go to.
Where's the fun, you may ask? Well, rather than an actual game mode, it would be something more to complement Legends mode—something you'd choose if you'd just like to get a feel for the layout of a certain area, or to see what has generated in the world. Recently I was looking around in Legends mode and found some guy who was entombed in a town somewhere. Once I found the town on a map (which took a while—maybe this could be made easier as well?) I decided to see if I could find his tomb. It took me several tries to get to the tomb, as every other time I was attacked by enemies on the way, or I drowned, or something. I just wanted to actually see him buried.
One last thing: it would be nice to be able to access this mode, and Legends mode, while you have a fortress or an adventurer in that world. If it could be considered cheating, just have it show the world as it was before you started that game.