The Nightstalker's Keen Scout perk is an extremely useful perk when hunting down chests and resources, but the target marking ability on it isn't the most useful thing since [b]only you[/b] can see the targets that have been marked. What would make this ability [b]SO[/b] much more useful, would be allowing marked targets to show up across your whole fireteam, allowing you to strategically point out hidden or important enemies to your whole team.
Just a suggestion I came up with while running around with my Nightstalker and enjoying TTK :)
EDIT: I am thinking [b]purely[/b] PvE, [b]NOT[/b] PvP. I agree with all of you, this perk would be way too overpowered in PvP if it was spread among your whole Fireteam. I was only thinking if it could be disabled during PvP.