Similar start to story id 10177155 Tricksters and Gods by sakurademonalchemist, as punishment for the alien invasion, Loki is stripped of his memories and powers and sent to be a mortal. There is a slight mishap, and he arrives in the past. Loki is born as Sirius. You may use the James is sterile and someone else is the dad plot. Preferably Sirius.
With Harry as Sirius/Loki's son, he is probably a metamorphagus, and while he was imitating James, Lily bound him to the form.
Preferably not slash
Let me know if/when you put up a story of this, and I will put the summary and story id on here.
I recomend that it starts moments after Sirius falls through the veil. (I.E. remebers he is Loki)
Thoughts behind this
Sirius doesn't get on with his family
Sirius is always up to mischief
Sirius once thought it would be funny to feed a classmate to his werewolf friend.