a rough idea I've had running through my head today:

sheith mer au where keith is a tiny betta mer being kept/shown off in a tiny filterless heaterless bowl on sendak's desk. sendak is a collector of all things rare and mythical and keith is one of his prize possessions.

through some series of events--likely involving blackmail and/or coercion--shiro ends up forced to work for sendak on a job, and the first time he enters sendak's office his eyes immediately zero in on the listless mer in the bowl on his desk.

sendak notices his attention and gloats about his beautiful jewel, tapping a huge finger against the glass until keith fans out his fins in fear and anger

long story short shiro devises a plan to steal the small colourful mer and brings him back to his place

he has a nice big tank with a filter and heater and lots of plants and ornaments and more space to swim than keith has ever known. the first few days were rough, keith was in bad shape after sendak's treatment, fins tattered and dull, too weak and sick to swim, barely able to eat

shiro had set the filter to the lowest flow and even that had been too strong a current for keith to deal with.

shiro was so worried he'd waited too long to get keith out and he hardly slept the first 48h. but eventually keith started regaining his strength.

his fins repaired themselves and regained their vivid colours, and keith was able to swim around the tank without trouble.

as he recovered his personality started to show and shiro was amused to find the little mer was endlessly curious, following shiro around the room,

watching him go about his days through the glass. keith seemed fascinated by shiro's tv (he had angled the screen when he'd noticed the mer watching over his shoulder), and shiro got the distinct impression the mer was mocking him every time he attempted to cook

when shiro would clean the tank keith would always initially dart away and hide, but soon he'd be investigating shiro's hands, swimming around just out of reach as if daring shiro to try and catch him. more than once shiro had turned what should've been a simple water change

into several hours of playing with the mer.

over time they learned to communicate in a way, a kind of simplified sign language that allowed them to understand each other. keith was very intelligent and witty and shiro found himself genuinely enjoying their banter

shiro nearly had a breakdown when he realized he was maybe possibly falling a little in love with the mer in his tank...