What about something that sticks with canon as far as late s3 - Tim saw a monster wearing Danny's skin, and years later found out that Sasha had died and been replaced by a monster months ago and he hadn't even noticed, so he admits that Jon's the only one he can trust to still be who he says he is, because Beholding's turning him into a different sort of monster and wouldn't let him get replaced.
Then it diverges: one of others gets close to Tim and manages to win his trust too, probably Melanie or Martin? He lets his guard down, starts sleeping with them and talking about his worries and plans. Then Jon finds out that the not!them escaped the tunnels and replaced Tim's new bff, proving it with tapes/polaroids/compulsion. How thoroughly would that break Tim? His trauma and anger and distrust all kicked up to another level by the revelation that someone he loves has been killed and replaced again, when he was already wary.
+ Tim lashes out at Jon for being the one to figure it out again. Because Jon's a monster, right? And Jon's still the only one he can believe is really who he says he is, not a malevolent imposter, which makes it safe to take it all out on him.
+ If Melanie's replaced, Georgie's the only one able to remember her and provide a description. If Martin's replaced, Jon's heartbroken too.
+ The not!them menaces them after they find out. Maybe they run to Daisy, or it could end any other way.
DNWs - Noncon before the not!them's discovered - though it could get violent afterwards (while taunting Tim about how he'd slept with it willingly last time), and/or Tim could assault someone afterwards (Jon or anyone who else who tries to say 'trust me') depending on how badly he reacts. - DNW Jonah getting any focus beyond a passing mention.
Prompt: Tim/not!them, fooled twice
Then it diverges: one of others gets close to Tim and manages to win his trust too, probably Melanie or Martin? He lets his guard down, starts sleeping with them and talking about his worries and plans. Then Jon finds out that the not!them escaped the tunnels and replaced Tim's new bff, proving it with tapes/polaroids/compulsion. How thoroughly would that break Tim? His trauma and anger and distrust all kicked up to another level by the revelation that someone he loves has been killed and replaced again, when he was already wary.
+ Tim lashes out at Jon for being the one to figure it out again. Because Jon's a monster, right? And Jon's still the only one he can believe is really who he says he is, not a malevolent imposter, which makes it safe to take it all out on him.
+ If Melanie's replaced, Georgie's the only one able to remember her and provide a description. If Martin's replaced, Jon's heartbroken too.
+ The not!them menaces them after they find out. Maybe they run to Daisy, or it could end any other way.
DNWs
- Noncon before the not!them's discovered - though it could get violent afterwards (while taunting Tim about how he'd slept with it willingly last time), and/or Tim could assault someone afterwards (Jon or anyone who else who tries to say 'trust me') depending on how badly he reacts.
- DNW Jonah getting any focus beyond a passing mention.