I may have a been watching way too much Darling in the FranxX but I got a way too good idea for a Mitsuru x Kokoro fanfic: Mitsuru comes from a slightly higher class family. He used to talk with the rest of the group but he slowly drifted away. Kokoro is a kind girl from a poor family that helps in the local shop for gardeners. She is the sunshine of the school the whole group attends and helps every student that needs help. Somehow, Mitsuru needs help with something and he ends up asking Kokoro to help. He then starts reconnecting with the rest of the group and he ends up going out with Kokoro. Somewhere along the lines somebody makes a comment about a pairing that nobody would have guessed worked.
For other characters, Zero Two would be a student that transferred lets say two years ago to their school and is going out with Hiro. Hiro is going out with 002. Ichigo got over her crush on Hiro and later started dating Goro. Koro is with Ichigo and is best friend of Hiro. Ikuno is also from a higher class family so she is in contact with Mitsuru. I am too lazy to write out the rest, just think it up.
Yeah... I thought this up in around 15 minutes... I am obsessed