Its a day late but better late than never right? 'Wanted to paticipate in the @sheithparticipationweek event so here goes nothing... a.k.a. a bunch of unrelated headcanons bunched up together...
•In the alternate universe with Sven and presumably Akira (this is still technically Sheith right??), its Sven/Shiro who constantly loses Akira/Keith. (I'm a sucker for Akira being under control of the evil!Alteans)
•Keith can sing really well (knows how to play the quitar too) and back at the Garrison, Shiro would ask him to sing for him once in a while. Keith was shy at first but he got confident the longer he sang for Shiro. A week before the Kerberos launch, Shiro asked Keith if he could record a few songs for him to take to space with him. Keith poured his heart and soul to the songs he sang (some he composed specifically for Shiro). Shiro spent his days travelling to Kerberos listening to them and remembering who he has to come back to after the mission.
•Keith is amazing at a lot of things...almost everything except swimming (or socializing as a matter of fact). He's sorta decent but for the life of him, he can't float. The only way he can swim is if he moves continuously which gets very tiring at some point. After Shiro's attempts of teaching him how to float ended up failing, Shiro had to become Keith's personal floater whenever they're swimming somewhere deep. Tbh, Shiro doesn't mind Keith clinging to him as they wade around a deep pool or ocean (somewhere where Keith can't reach to bottom height-wise). Shiro isn't mean about it as well, he doesn't do those pranks where he pretends to let Keith fall or anything like that... he genuinely cares to make Keith feel secure despite not being comfortable around the water. (Keith gets the hang of floating... eventually.)
•They're married. (They just are, hush child, they just are)