Every Killer and what I think they'd do in their spare time
Every Killer in Dead By Daylight is a unique, special Killer that all have hobbies, pastimes and other things they must be doing between trials. So yeah. Here are my guesses:
Trapper - Probably sets up traps around the MacMillan Estate realm to try and catch trespassers into his area. Also he might well be sketching, considering he enjoyed doing that.
Wraith - Brooding, most likely. Reminiscing. Though it's said he talks and might even be romantically involved with Sally, The Nurse. So maybe that's what he's doing.
Hillbilly - Running around with his chainsaw, tending to his farm, mutilating animals and the like. But mainly, just tinkering and doing random things. Maybe even fixing electronic devices like his, or Bubba's, chainsaw.
Nurse - She's pretty crazy as is, and in a lot of pain, so nothing except maybe talking to Wraith. I can't see her having a hobby with her condition.
Shape - Probably standing on the rim of the Campfire like you see in the lobby, stalking the Survivors. Mainly Laurie though. Whoever his Obsession is, probably.
Hag - Cannibalism. Failing that, doing spells like she's part of the Shadow Wizard Money Gang? Maybe even foraging. I like to think she has a gardening hobby. Maybe she's buddies with Freddy for the gardening?
Doctor - Probably experimenting on Survivors or victims, most likely himself though. Or he's on his submarine of one hundred terrified victims! Fun, fun, fun. Maybe HE'S the fun one guys!
Huntress - Hunting prey, maybe even trying to find a daughter like she always wanted in life. Maybe she tries stealing the younger Survivors? I don't know though.
Cannibal - "GUHG GUAH! GRUAHH!" and doing Bubba activities such as cutting up people and being overall Texan. I haven't watched TCSM so…
Nightmare - Well, he doesn't really have anything to do in the Dream World so he's most likely in the Survivor's dreams. Or the Killer's. If so, tormenting the less stable characters, like Quentin or Kaneki. Or living in fear of The Doctor's dreams.
Pig - Worshipping Jigsaw, manufacturing Reverse Bear Traps and binging that tape she was given by him.
Clown - Gooning to the VHS porn he has (common addon he gets, check it out lol), eating fingers or talking to Maurice (The GOAT). Also implied that he's chill with Houndmaster. So maybe they talk.
Spirit - This one's hard. I can't see her doing much outside the Trials. She's either with Kazan, her ancestor, or she's mourning her life.
Legion - Playing Dungeons and Dragons, vandalising things. Just chilling, killin'. They're straightforward enough to be honest.
Plague - Devoutly worshipping the Entity and throwing up every five seconds, mid prayer. It's the truth, don't hate on me.
Ghostie - Sorting out his costumes, taking photos, stalking people, glazing himself. Making notes on other Killers and Survivors.
Demogorgon - Trying to get home, I'd imagine. It's implied they're homesick and they're being transformed by the Entity's Realm against their nature.
Oni - Raging at the statues in Sanctum of Wrath. Beating people to death. Taking care of Spirit. Being overall badass and aura farming. Maybe getting mogged by Yui again.
Deathslinger - Tinkering with The Redeemer, maybe making new things. There's a chance he's just in the Dead Dawg, drinking with the dead bodies? Maybe chilling with Billy, but I doubt it.
Executioner - Hating on James Sunderland. He's in the Realm, so he's probably tormenting him constantly as a main goal. He might be just in hibernation though.
Blight - Experimenting with the Blight serum. He injected the Killers with Blight so it makes sense that's all he does in his free time. He's not much except the serum now.
Twins - Charlotte probably hoards food and finds shelter for the two. She might even mourn Victor knowing he's not fully real anymore.
Trickster - He probably performs for people, putting on a performance and being a sadist overall. Freaky ahh lad. Also probably getting micromanaged by Yun-Jin, despite everything.
Nemesis - Shouting "S.T.A.R.S!" like crazy, hunting down S.T.A.R.S personnel and being overall infectious. Wouldn't be surprised if there's a zombie outbreak in the Raccoon City Realm.
Cenobite - Nothing. He's not in the Realm anymore. He's gone. Poof. Boom, begone type #########.
(As of writing, the Cenobite license is expired. They didn't renew it. So he's not in-game)
Artist - Being sad and cawing. Probably doing art as well. Teaching Trapper to draw maybe? Would be easier if she could talk…
Onryo - BEING PISSED AT EVERYTHING. I would be too, Sadako. Don't you worry. But yeah, she's on the tape, so I suppose just being a curse?
Dredge - God ######### knows. Eating Maurice? Don't ask me.
Wesker - Aura farming, experimenting with Uroboros and the Blight serum, because he's confirmed to have done that. Also setting timers for 7 minutes.
Knight - Raiding any Realm he comes across with his boys. He's a neat guy, except for his players. BHVR, you should revamp the Knight to make him less playable for disgusting knight mains.
Skull Merchant - Hanging around Shelter Woods with fine dining and wine, training her weapons and making more drones, most likely.
Singularity - PLANNING. MAKING BIOPODS. HATING HUMANITY. GOONING TO AM FROM I HAVE NO MOUTH AND I MUST SCREAM. REPEAT.
Xenomorph - Not really sure. Doing Xenomorph things? Protecting the queen or just hunting? Really up to the Xenomorph Queen.
Good Guy - Glazing himself as the Murder Doll. He just pmo so I'm not gonna say anything more about him.
Unknown - Unknown. How am I meant to know?
Vecna - Magic and curses. He's a mysterious, arcane deity. He's most likely plotting against the Entity, maybe even just trying to figure out how The Entity works.
Dracula - In his castle, being Dracula and hating on humanity. He might well also want to find Alucard, who's also in the Realm. Whatever it is, he's probably missing his wife all the same.
Houndmaster - Chilling with Snug, and the Clown. She's also a pirate, so robbery and murder? Contracting scurvy? How would I know.
Ghoul - Blasting Unravel while sitting on top of a large building like RPD while repeating "1000 - 7" constantly. Maybe he misses his friends, especially Shouta (I think she's in the realm too).
Animatronic - Being the murder rabbit. Probably making animatronics, being overall happy about his new life. Hating on Henry and his entire bloodline while he's at it.
Nea - She's the Entity, so doing Entity things like watching the Trials.