Display a list of mobs currently targeting anyone other than any raid member marked as a tank.
Check nameplates to get the names and targets of all nearby mobs, (Tidy Plates shows this as possible)
Check raid member targets for any additional mobs not close enough. Eliminate duplicates based on health and targets.
Possibly allow for the addon to report to other instances of itself, run by other raid members, any loose mobs to catch those whose nameplates might not be in range of the tank.
I am considering developing an addon along these lines. Does anyone know if it already exists? On fights like Maloriak it would be nice to get an instant report when an add turns to attack a healer.
It would not be able to help you target the loose mobs, only see that there are loose mobs, how many, and their names.