In one of my fanfics, I went with Cawlin x Peatrice.
The reasoning was that they were both lonely souls who'd been jilted by the people they originally loved (Karane dumping Cawlin for Pipit and Link dumping Peatrice if you play the game that way). I figured they could, in their loneliness and grumbling, find each other and try to make the best of it.
Or I just smooshed them together because I felt sorry for the both of them.