I wonder why this idea wasn't put in the game already? It's a no brainer.
You take the bosses, you put them in a giant arena and the players beat 'em up. What's not to love?
Players could fight the regular bosses, story bosses and even the shadow lair bosses, and maybe add two modes:
Regular Boss Rush: Where players recover health after every fight.
and Hardcore Boss Rush: Where players fight each boss in a row with no break room in between, if players feel suicidal.
Make it so players don't have to switch weapons in break rooms, so setting up a strategy is a must when on a team. (If everyone equips the fastest weapon to kill each boss, it would ruin the fun.)
And instead of winning each boss token, players can win Krogmo Coins (because Lockdown nowadays takes forever to play on a team, more if half the time you can't hit anyone because of lag issues.)
Also, buff the bosses a little bit, so it doesn't take two seconds to kill them.