Me and my usual group of people I game with have made a collection of names / noises to refer to all the mercs, not exactly nicknames, but for some reason these names and damn right stupid sounds have stuck for us.
Arty: Arthur, (I don’t know why, but it stuck)
Aura: Aura the meeedic (said in an exaggerated southern American accent)
Bushwacker: Wack-a-bush
Fletcher: OP sticky bomb c*** / Sir need-a-nerf
Fragger: Fragenstein
Kira: Laser Light Yagami
Nader: Little Eggs of Dess!
Phantom: Phanthomas / Spooky Scary
Phoenix: El Médico
Proxy: Prox / Shotgun Slut
Red Eye: Pink Eye
Rhino: Fat F*** / Big Baby Neil (why Neil? not a clue I’m afraid)
Sawbones: (don’t really have a name for him, we just mimic his voice badly instead)
Skyhammer: Mrs Nesbit (don’t even ask)
Sparks: Sporks
Stoker: Toker
Vasilli: Vagisil (Original, I know)
Not Exactly creative, but felt I should share 