Option 1: Make your character hold it sideways like a "gangster".
Option 2: Thorn already has a distinctive firing noise. Make it do Omnigul's shriek when you reload it.
Option 3: If you've played GTAV, you might know about the mechanic where you yell into your mic to intimidate store clerks. Well they could make it so you have to compliment your Thorn every kill or it will start to dislike you.
Option 4: Mandatory pink camo.
Option 5: Have actual thorns on the gun. It wouldn't affect gunplay, but your character would occasionally "Oww!"