Oo what about Ethan/Bianca !
Again, I love Ethan, and I really really like Bianca too (without her being one of my favorites characters, but still).
Ethan needs friends, and a family. I mean... can you imagine Ethan being apart of the underworld children family !? Him bonding with Nico ? Hazel ?! Just... yes !
If Bianca had survived I could see her trying to be friend with Ethan, she's forgiving, nice and wants to protect people she loves and Ethan needs love.
Ethan could teach her how to spare, and I picture them maybe being a little put aside by the campers after the second Titan War. So that's how they could learn more about each other.
Or, otherwise, there's the sadder possibility of them encountering in the Underworld, if Bianca didn't choose to rebirth. Maybe sad but nice if you imagine all of the dead characters being friends in the afterlife, so still cool for me.
I really like this pairing, there's clearly nothing to retire for it in the canon, but with a little imagination it's one of the most interesting fanon pairing ever !