Chapter 2
After Nico ran away he started to learn how to shadow travel and had a few mishaps that landed him in China. What if he accidentally ended up in London outside the Leakey Couldron and passed out. Tom the bartender took him inside and put him in a room, and when Nico wakes up he finds out about the Wizarding world. Since he's eleven, and it's a weird case, he gets accepted into Hogwarts in the middle of the year (it's like February or something, I don't know when school starts after Christmas break). He goes to Hogwarts and gets Sortet into Hufflepuff (this is before his complete personality change after spending years alone hating himself, so he can still go back to being the kid he was before Bianca died if someone helps him move on). Cedric Diggory takes him under his wing and he makes new friends that love him and give him support. (I hc this as the year before Harry and the gang arrive so he is a year younger than the twins and a year older than Harry. This also obviously does not follow the time line otherwise he would have traveled back in time)
IDK I have seen things like this before but never completed ones and never where he I'd a Hufflepuff. If anyone uses this idea you can decide the impact he has on the plot of the books and everything. If you continue this into the HOO series (everything further in PJO happens in the summer so it doesn't affect his school years) he tells Cedric about demigods (I see them as like brothers and he takes on the role Bianca had) and Cedric knows about the wars and stuff so they come up with a story to tell everyone why he missed so much school. (He starts his second year after the BotL, his third year after TLO and the HOO series is set during the end of his third year, summer break and the start of his fourth year, which would be The Prisoner of Azkaban.) You can also add a quest during The Goblet of Fire or the Order of the Phoenix if you want where demigods (you can choose who) is sent to Hogwarts to assist Harry in defeating Voldemort and they find Nico there and he helps them with finding and destroying the horcruxes. You can choose the ships if you wnat any just keep them as realistic as possible and try to not go OOC with anyone.