MC will be reborn as a newly born Kasugai crow who had been bred by the master himself (Ubuyashiki).
MC is taken care of and taught by the master himself, developing a close relationship.
The timeline is more or less a few years before cannon starts.
Write about learning how to speak and messenger crow duties, as well as interactions with the master.
Write about the bond between mc and Ubuyashiki.
Write about interacting with other crows and maybe the Hashira's and their crows.
The mc could be assigned to some random npc slayer at first, but gets killed, then gets assigned to a slayer who gets to a higher position, or you could make it up yourself.
That slayer could either get killed again or you could make him/her a brand new oc hashira.
If the second slayer dies, mc could be known as a 'cursed crow' and stay at Ubuyashiki's mansion instead.
Write about when canon starts with the hashira meeting, mc is present and is assigned to watch over Tanjiro and Nezuko.
Write about the Ubuyashiki mansion explosion, with mc witnessing his master's death.
Write about either dying in the explosion as well or watching over Tanjiro after the Infinity Castle arc as per his/her master's wish.