Here are some 5SOS fan fictions that are really good. You can find lots more on Wattpad. :)
Unpredictable Calum Hood Fan Fic
http://www.wattpad.com/story/7216815-unpredictable-calum-hood
Fall Luke Hemmings Fan Fic
http://www.wattpad.com/story/9468909-fall-luke-hemmings
This is me 5SOS
http://www.wattpad.com/story/7812756-this-is-me~-5sos
Complicated Luke Hemmings Fan Fic
http://www.wattpad.com/story/9876665-complicated-luke-hemmings
She's The Man 5SOS
http://www.wattpad.com/story/7325584-she%27s-the-man-5sos-~-watty-awards-2013
She's The Mom (squeal to She's The Man but it is written by another author) http://www.wattpad.com/story/9008763-she%27s-the-mom
I Need You Ashton Irwin Fan Fic
http://www.wattpad.com/story/8608607-i-need-you-ashton-irwin
That Song Luke Hemmings Fan Fic
http://www.wattpad.com/story/4846351-that-song-5sos-luke-hemmings
We will update you on more Fan Fics in the future :))) x