What fanfic couples are there for Katekyo Hitman Reborn? I saw this picture with Adult Lambo and Adult reborn

Woh! Many.
Mostly Tsuna with any KHR character but there are also Hibari x Mukuro, Reborn x Lambo (both adult of course XD), Gokudera x Yamamoto, Squalo x Xanxus, Mukuro x Byakuran, Giotto x G., Hibari x Yamamoto, Gaokudera x Haru, Hibari x Chrome and so on.
Practically any pairing you want. ^^

idk. But here is a picture to fill your imagination. by ~xx-Clouds-Over-Luna on deviantart.