3(Katekyo Hitman Reborn)

An Au where Nana comes from a noble family and the only reason she was allowed to marry Iemitsu was because she was the youngest child of six so she could choose who to marry but by the time Tsuna was born most of Nana's siblings had died or been married off to another family.

The only male left of Nana's family was her grandfather who had to approach Nana when Tsuna was around 6(after the seal) and tell her that he would name Tsuna has his heir and would let Nana's grandmother choose a good future spouse for Tsuna.

While Nana wished that her child could also choose who to marry in the future, she understood her family and since Iemitsu was from a middle class family, this was a sacrifice Tsuna had to do as his father's family had no power over his future(maybe Nana's family is even older then the Vongola family but doesn't really has a connection with mafia/yakuza/etc).

So, Nana talked with her son and informed Tsuna that he would start spending the weeks with her family and the weekends with her but that she still loved him and was not abandoning him.

And with that started Tsuna's training as the Kokushibyo(this means 'Black death' but the name of the family can be changed by the writer) family's heir which slowly broke the seal that Timoteo put on him.

Tsuna was adored by everyone in the Kokushibyo family but much more by his grandmother, Schadel Kokushibyo(nee Tod)(Schadel means Skull and Tod means Death), who whenever she could would spoil Tsuna.

(And yes, I'm making Skull de Mort as a woman, Nana's mother and Tsuna's grandmother because Skull is my fav but you can make Tsuna's grandmother an oc or other character).

It was as Tsuna turned 14(before Reborn arrived), that Schadel made a choice of the perfect future spouse for Tsuna(it is up to the writer who it is, it can be a character from the anime/show as long as it isn't an arcobaleno or it can be an oc).

By the time Reborn came to Japan, Tsuna was already trained to be a leader and helped his grandfather with business sometimes during the week besides already having a complete/or almost complete set of guardians.

What happens after this, how this affects Vongola, what ships it will have, etc is up to the writer.