it started out straight and then the girl died only to be reincarnated, as a boy.
He only remembers his past life as a series of dreams? The god fell for her, but then because of some accident, he caused, she died and he vowed to never fall for a human again because of that...only for many years to pass and he finds himself falling for a boy that reminds him so much of his lost love. Much later on he finds out, after several hard test the both had to go through, that the boy he has placed his affections with is the reincarnation of his past love.

(Kind of similar in plot line-ish to The manga/anime the betrayal knows my name. )

A woman is cursed with "the siren's curse" that wipes your memories of your past and turns you into a siren. Usually happens to heart broken people who turn to the ocean for suicide but get a second chance to live by becoming a siren. In return the siren to offer the trade becomes human.
This idea is based off a story about an accident that splits the curse between two people because of an unrequited love situation. That leads to a search to break the curse for all the sirens eventually, while not being consumed by the curse.

Raised as the heir "male" of the family because of her mother that wanted a boy, but couldn't have any more kids after her brother was born early. The brother is too sick to take over? Daughter is sent to an all male boarding school for the gifted to learn how to take over the family business. She has to hide her gender from the school and her maybe roommate(s)?

two romance options:
1) roommate falls for "him" then finds out that he is a she.
2) girl joins an illegal fight club to raise money for her bother's hospital bills, teacher becomes her partner -> falls for "him" trough all the fights they do, has a mild gay panic but finds out his partner's true gender by accident and has to come to terms that ah they have been female all this time.