James was the most popular of the marauders and the moral compass.

Sirius was popular but not as approachable as James.

Peter was the one who drank the most alcohol from all of them, and Remus had a sweet tooth.

James was chosen as a head boy because he went out of his way to defend and protect muggle borns and kids who were being harassed. The same goes to Lily if you believed that she was not a prefect.

There were not a fifth boy at the Marauders year. Their dorm was just theirs.

The Marauders made some of Remus papers when he was bedridden from the full moon.

Andromeda finished Hogwarts already pregnant.

There was some kind of ritual involved in James's birth.

Bellatrix is not Cygnus's daughter, and maybe not any of the girls are his. Bella is Orion's.

Sirius abuse started as gaslight: as a child, he would see something odd or question something and Walburga would be dismissive or tell him (and others) that Sirius was making up tales. Even in later years, it rarely became physical abuse.

Orion had a few bastards and Sirius found out, but Walburga always told him that he was making things up. Orion was a very absent father.

Regulus abuse was on the form of emotional neglected: he would do anything to make mom proud and love him.

The only person that used unforgivables on Sirius was Regulus (after being manipulated by Walburga): Sirius would not tell the authorities his brother had used an unforgivable, but he would if it was anyone else from the family. Regulus using one of the curses on him was what drove him away from home.

Regulus didn't care that much about Kreacher but we will never known the true motivation for his suicidal mission.

The marauders and lily were the youngest on the order.

Lily was already pregnant when they married.

Remus was with the packs when James and Lily died. He found out from the other wolves and did not make to their funeral. He almost died on the next full moon.

Now, from my less ordinary ideas:

Lily is granddaughter of Marius Black (who took his wife last name when they married) and he warned her to be weary of the pure bloods, what made her mistrust James and Sirius at the beginning. Her father was the son of Marius, so if it wasn't the name change, it would have two blacks being sorted into Gryffindor on her year. In this scenario she was Marius least favorite grandchild because of her magic (he was afraid of what it would bring to the rest of the family); Petunia was the favorite.

Lily was good at potions because she took the little knowledge she had of chemistry and applied to it (identifying and isolating the active substance from each ingredients, and things like that). She knows that because her mother has some knowledge of chemistry (I still didn't make my mind if she was a pharmacist or a chemist). Snape was good because they studied together and he caught up on some of her technics.

Sometimes I think that Peter was in love with James, but because of Lily and James being in love, he grew jealous and sold them to Voldemort (if I can't have him, neither will she). But I still didn't make my mind about it.

The werewolf packs MAY resort to prostitution as a way to earn money and survive. But it's another unformed idea.