This could be achieved by adding "seen by" above the "Advanced" tab "Player Permission" options. The way Player Permissions worked would not have to change. The only difference would be the helper text " See: All Players can view " to "See: Selected Players can view". It may be nice to have "Seen By" default to all players. If you wanted to get fancier, on the tokens for PCs/NPCs, add information for what they can detect. For example: normal, scent, invisible, extra planar, etc. These detect types could be generated by the GM. Then, on the tokens, have checkboxes for the various methods of being detected. If your PC token doesn't have the right kind of detect to "see" the token in question, then they do not see it. This feature would have many applications in many gaming systems where for various reasons people or objects may not be seen by everyone in the group.
