AU/team dynamic switch-up! Joe and Nicky did not live/die in the 1000s. The Old Guard rescue them as mortal kids/teens while on a mission. Joe and Nicky are competent/endearing during the rescue and also witness that the Old Guard are immortal. The Old Guard rarely take in children (because it's heartbreaking to outlive them) but Booker begs Andy to let Joe and Nicky stay with them as they have nowhere else to go. Booker is still grieving the death of his sons and maybe wants to help these two since they already know he's immortal and so can't resent him for it.

The team raises Joe and Nicky (but not as siblings since it's clear from the off that these two kids are sweet on each other), so Joe and Nicky grow up together as childhood sweethearts, learning how to fight/speak more languages etc. from their found family. But because they are mortal they are frustrated not to be fully involved in the missions and by the family's over-protectiveness.
But when they are in their 30s they die (one first or both together?) and the family mourns but then...guess what?? They're immortal!

- Bonus for Nile joining the Old Guard and not understanding why Joe and Nicky look a similar age to the others but refer to them sometimes like they are their parents/aunties.
Booker isn't looking for a way out of immortality! He's a cool French Dad! Yey!
- Quynh maybe got rescued early from the iron maiden? Because I need to see Auntie Quynh and Auntie Andy teaching Joe and Nicky how to fight.