5(Katekyo Hitman Reborn)

An Au where Iemitsu and Nana divorced when Tsuna was around three as while Nana loved her husband, she thought he would take a break from his job and help her with their son but it seemed like he stayed away longer so she decided that it was for the best if she divorced him if she was to be a single mother no matter if she was married or not.

After the divorce, Nana moved back into Namimori(before they lived in another city) with her brother-in-law Yamamoto Tsuyoshi(so Nana and Takeshi's mother are siblings which make their children cousins) who also needed help with his own son since Nana's sister got sick and later they found out she had cancer.

In the hospital, Nana's sister met a young lady named Lavina who was also sick and had a child so Nana's sister offered Nana as a baby sitter for Lavina's child(Lavina would pay Nana to babysit Hayato who was being raised by his mother and not father and stepmother...Lavina did not tell Hayato's father that she was pregnant and moved back to Japan) who was really thankful for the offer especially for when she needed to go to the hospital or had to work on her job as a piano teacher.

One day as Nana was taking the kids to a park, she accidentally got the attention of an Italian man who had been visiting his cousin with his own baby as a way to cope with the death of his wife. The man, Lorenzo Bovino, approached Nana to talk with her mostly to ask for advices on how to take care of his son as he had heard a few woman bad mouth Nana about being a single mother. The two talked the whole time Hayato, Takeshi and Tsuna played in the park with Lambo peacefully sleeping most of the time just waking once because he was hungry.

Seeing that Lorenzo could do with a babysitter and someone who understood him(even if Nana was divorced and not a widow), she offered to help him with Lambo and anything he may need which Lorenzo accepted.

Lorenzo's cousin, Hibari Alessandra(nee Bovino), was happy for her cousin and seeing how well the children under Nana's care got along she decided to force her son(Kyoya) to hang out with them to see if he would come out of his shell and not be so...animalistic/violent.

At some point, when Kyoya joined the other kids he was followed by three other ones(Ryohei, Kyoko and Hana) who while unexpected were welcome to the group and Nana even talked with their parents so they wouldn't worry that their children been kidnapped.

When Kyoya and Ryohei  were around 10(they are all closer in age in this Au), Lorenzo and Lavina(who together with Tsuyoshi's wife survived their illness because Lorenzo got them help from the best sun flame user he could get his hands on and from Shamal) brought new kids into their group with one being a little girl Lavina saved from her abusive and neglectful parents and the other being a group of four(Mukuro, Ken, Chikusa and M.M) who Lorenzo was able to rescue from a bad famiglia(because I forgot their name xD).

Of course that those 5 had a harder time integrating themselves in the group but soon it was like they had always been part of their small family like all the others.

How would this affect the canon events? They all know about mafia as Lorenzo would tell them about it so they knew what accepting him into their group would bring(Lorenzo, Tsuyoshi, the Hibari clan and the Bovino famiglia trained everyone at least in self-defense and how to to protect themselves in case their flames became active and some famiglia tried to force them to harmonise), there wouldn't be the Dame-Tsuna thing as his flames wouldn't be sealed(so Tsuna would be a normal kid with mostly average notes), Vongola(including Iemitsu) knows of Tsuna but know nothing about him and the things that happen around him and Nana, I was thinking Nana would have later married Lorenzo(especially as he proved to be a much better husband and father then Iemitsu even if both were mafia since Lorenzo made time for Nana and the children instead of disappearing for years), Reborn has no actual information about Tsuna as Skull owed a favor to Tsuyoshi and got Vipper to help hide what was happening in Namimori(and it helped that Kawahira had a soft spot for this weird found family and helped keep most mafiosi out of Namimori) and having adults they trust would probably change how a few events would go including Tsuna asking Lorenzo's help when he found out he was now the Vongola heir(so he would know about Xanxus and Timoteo's actions much sooner plus other things) or them not trusting Reborn or others so easily(look I know they made Nana and other background characters oblivious but if someone heard an explosion...there would be people calling the police/ambulance or go see what happened and really why would Nana trust Reborn so easily even if she loved Iemitsu it doesn't translate to her easily opening her home for a baby looking man who she knew nothing about...what if Reborn had been a predator or something like that?), if Bianchi and her parents ever find out about Hayato and Lavina would be up to the writer but I like the take of Lavina ending up with both(Hayato's father and Bianchi's mother), Takeshi wouldn't be so carefree(he, Kyoko and a few otehrs may act as such but it is all an act), etc.

Anyways, any other change is up to the writer including the possibility of ships and how each arc would end up with all this changes.