hi dearest anon!!!! oooh what a DELICIOUS question thank you! the Case universe for me exists in this eternal earlier stage of seungin's bdsm exploration so the below is an example of 'a year or two later' not a 'decade' later (seung has a pup hood by then i think and they are SO established and comfortable in their play that they can slip in and out of it effortlessly)

they take a while to build up to 'crazier' things, moving past the stop-light system once they know each other well enough, and after jeongin buys seungmin's accessories. they get seungmin a tail plug, and eventually a home cage lined with lovely pillows that he becomes really fond of. the most intense session happens when jeongin gets jealous of the /cage/ because seungmin starts to nap in there which was hot initially but then it means that they aren't spooning on the couch or cuddling on the bed, so jeongin leashes him while he's asleep and wakes him right into a scene, tugging him out into the living room while seungmin blinks and adjusts to the sudden drop. and then, something they've never done before (on purpose): jeongin has seung get fully dressed and takes off his leash and tucks the collar under seungmin's high-necked shirt so he can go out in subspace in public. they go about a regular day; grocery shop, return a package, pick up takeaway, all while seungmin is completely floating and entirely at jeongin's mercy. no one around them can tell, of course, since the goal is to be 100% inconspicuous and not involve unwilling participants in their play, but seungmin gets increasingly floaty and out of it and high for hours, their longest time with him in subspace yet, and the first time he purposefully goes outside in that state. he can't speak, so jeongin has to do all the talking for him, but that feels good too, and things build between them all day. eventually, jeongin leads him back home where he puts him through a really intense session to release all the built-up energy. it involves seung having to do 'tricks' (he has to come on command, then lick it up to keep the floor clean, he has to let jeongin overstimulate him, make him come again and again while he barks and whines and does everything jeongin asks at the same time), earning 'treats' (getting to sniff jeongin or suck his dick or kiss him, depending on how well he's done) and wearing his tail plug which jeongin pulls to 'punish him' (it feels amazing and seung loves it). it ends with choking while jeongin fucks him, which seungmin loves but jeongin reserves for VERY special occasions, leaving him a melting, drooling mess on the bed and sooooo so so happy