Create a full map view, and radar trinket item that has cumulative benefits based on how many group members are using it.

add the option for a key that would overlay a semi transparent map of the current level, showing all reveled sections of the map.

If you have the "radar module" you are able to see an outline of the entire level, with a fog or color indicator of where has been discovered thus far. In addition you can see:
(if 2 people have the module) party buttons, start and end gate, as well as locks and keys
(if 3 people have the module) near by hidden buttons (under bushes and boxes) opened and closed gates, switches, etc
(if all 4 people have the module)show all spawned monsters, especially those darn phantoms that like to follow you the whole level.

So what do you think?