We do already have an icon for unreachable houses, but I think an icon for unreachable build sites would be nice. Too often (especially in hilly terrain) do I issue commands to flatten lands or build walls and no builder takes up the job. It is pretty much guesswork at this point. Sometimes, reachability is not even the issue. But all I can do is to wait, disable and enable the task, and hope that someone will pick the job up.
A notification for trapped villagers (if they can’t pathfind to markets, wells, and their house) would also be nice. Oftentimes, villagers build the walls from the unreachable side, and get trapped after completion. They can also get trapped after flattening land in hilly areas. Sometimes, they break their tools and can’t dig their way out. (Yes, I build in hilly areas a lot, and I do not think the existing tools are adequate, especially the camera can be annoying).