Here is a 3.5/4th edition feat for today
Gaining the Upper Hand after killing a non-minion enemy and reducing the total number of conscious enemies to below your number of conscious allies, make an intimidate check as a swift/minor action. This check sets the DC for the sense motive check all enemies must make to avoid becoming frightened or freeing (DM choice). Mindless enemies are immune.
Throat Punch Games 
Not very familiar with the minion mechanics for 4th but as it stands, it may need a more hit dice/level based formula for it to work. Also you might want to consider making this a teamwork feat. It seems a bit much for just one PC to have and give the benefit to the entire group (maybe at least two people in the party must have it, one of which has to deal the killing blow). One last note, fear effects are cumulative. Something to keep in mind when deciding what type of feat and/or any additional prerequisites it may need to balance it out.