I don't know how to do that, or I would surely have a Victory emote. 
Give me a suggestion, how does one create such a macro?
This works because the enemy you just dropped takes awhile to fade. So as long as it's still around, you can base a macro off of it.
So it's pretty much just a standard saying/emote:
/bind alt+v "say Hey $target, how's that floor taste, huh?$$em taunt"
I asked pretty much because of the roleplay elements... what does your char say when he or she drops a particularly tough opponent?
(you could use this in the Arena too.)
Do be careful, because the game doesn't know the difference between friendly and hostile targets. so if you're assisting a lot, you might want to not do this. People tend to misunderstand when you trash-talk them, instead of the enemy.
"City of Heroes. April 27, 2004 - August 31, 2012. Obliterated not with a weapon of mass destruction, not by an all-powerful supervillain... but by a cold-hearted and cowardly corporate suck-up."
This isn't predefined, but occasionally people like to do this when they beat down a boss or an AV.
Soullll Trainnn: "$target, $name just took you downtown... beeeyotch!"
Multiball Bonus: "Game over, no replay awarded."
Doc Mayhem: /em bow "You were a ... worthy ... opponent."
Mistress Ohm: "Oh I'm sorry, was that too much juice for you?"
"City of Heroes. April 27, 2004 - August 31, 2012. Obliterated not with a weapon of mass destruction, not by an all-powerful supervillain... but by a cold-hearted and cowardly corporate suck-up."