First, my use of “fast-travel middle ground” was never meant to imply anything like the harbors. With my idea the amount of time to travel from place to place would still be the same, however, you as a player simply would not have to be present the entire time.
The stagecoach would offer folks the chance to rest their eyes, use the restroom, and grab a drink or a snack and so on.
Some thoughts:
Should be attackable, absolutely. How this might work (if it is an automated system) is that the game uses each characters stats to figure out how they fared in the attack and whether or not the lost any or all of their personal items, including gold if they are carrying any. I don’t think that equipped armor or weapons should be at risk. These are my ideas; what are yours?
The stagecoach should be of limited access. Only one per city and once it travels out it must travel back again before it is once again available. This will prevent everyone from traveling this way constantly.
I am still undecided about whether players could or should drive them or if it should be done strictly by NPCs. Like I said in my first post though this is just a very rough idea.