Kono Ore Ga Omae Nanka Suki Na Wakenai
When the boy who rejected him in middle school joins their class - and worse, doesn't even remember him - Fujima decides to make him fall in love and reject him as payback. But then he remembers why he loved him in the first place.
Hana No Miyako De
This ties into Hana no Mizo Shiru and tells the professor and Misaki's grandfather's story. It will probably make you cry. Misaki and Arikawa are in this as well, and we get a little relationship development to make up for the tragedy in the first chapter. ETA: Maybe they saw one another while Akira was still alive, after his wife had passed. I like to think they wrote each other letters and remained good friends, and maybe even stole away for time together sometimes, hopefully with the full knowledge of their partners.
Hishakaishindo
A musician's natural talent costs him chances at friendship, so he stops playing completely and walls himself off to the world, until a photographer befriends him on a rooftop. But once he falls in love, he runs away. Three years later, they're reunited when the photographer is hired to photograph his new band.
Yamada To Shounen
One evening in the snow, a 26yo man finds a high school student passed out drunk and takes him home. Over the next year, they become friends and fall in love. The author published extras that make up the 7th chapter. She envisions them together in a decade. I love how much she loves them!
Love Nest (Minaduki Yuu)
Oh my god they were roommates! A well-meaning, meddling friend matchmakes a troubled 20-something and a 38-year-old divorcee. The matchmaker is also 38 and has his own spinoff called Engage.
Free! Dj - Never Let You Know