One of my favorite custom variants me and my lan buddies play is called "Camp Whores". Starting weapon is sniper, secondary is alien sniper. No shields, and you need 50 kills to win, any amount of people. BUT, there is a catch, the only level we can play this on is Ascension, without the banshee. Sumtimes we play Zanzibar, but mostly Ascension.
Another one is by my bro, he made up "O Malley Madness". O Malley is a character off of Red vs. Blue. 250 kills is needed to win, team slayer rules, in Lockout only. Startout weapon is pistol, and secondary is bubble gun (plasma pistol). Random weapons needs to be on, for scattered weapons on the map. This variant is quite fun, since it is a tight level.