In addition to the controlled by and such add a dropdown like the handouts use of "can be seen by" for players on tokens. Not sure how much work of the engine that would take to create as it may change the way the tabletop is drawn. Maybe a simple "if 'can see', show token image, if not show a default of a 1x1 transparent token (basically invisible to them)" But for powers like see invisible, or perception checks it would be an immense help if say only players X,Y,Z could see said token.