Westeros Soulmates AU. There may eventually be a few child settings under this.
To start off, this original sub-setting features a soulmate system where children who have soulmates are either born with - if their soulmate is already alive - or acquire - if their soulmate is born after them - monochromatic eyes. They have these until they lock eyes with their soulmate for the first time, at which point their eyes will colour. There are subtle ways to tell if someone is soul bonded, which Maesters and other learned people know.
Many nobles, particularly in the south (though not all!), will hide the fact that their child or dependent is soulmated, so they can use them in marriage alliances. In the North and in Dorne it is less acceptable to do this. Honour duels may be fought to claim a soulmate who is married to another, or whose parent refuses to allow them to marry their mate or join their household. Women may name a champion to do this.
F/F and M/M pairings are usually seen as platonic. F/F soulmates will usually see the woman of higher status take the woman of lower status as a handmaid or other dependent of some kind. M/M soulmates will usually see the man of higher status take the man of lower status into his household as well, as a sworn shield, guard, advisor, etc. Often one of the men of a M/M pairing might become a Maester.
This is all among nobles, of course. Roughly 1/10 people will have a soulmate at some point in their life. Many halves of a soulmate pair will die before a meeting can occur. Most soulmates are peasants, but it's probably the case that most soulmates who meet are nobles.