Picture a world that was strange and exciting, yet perfect, unravaged by war.
Tom Marvolo Riddle did not go Dark, and therefore, the thing that was known as Lord Voldemort never existed, and the world never knew what could have happened.
As there was no Voldemort, Severus Snape did not go Dark, and, due to spending long periods of time with Lily Evans, disaproved of the things his housemates did, despite his Slytherin heritage. James Potter tolerated Severus once he got Lily to tolerate him, so he wouldn't ruin their friendship. The two were never best friends, but they could stay in the same room without hexing each other. Sometimes.
Once Lily was sure that James' head was actually de-shrunk, she agreed to go on a trip to Hogsmeade around Valentines day, and they started dating.
The war never happened. Frank Longbottom married Alice.
James married Lily.
As there was no Voldemort, Peter never went Dark. The Marauders were still the best of friends. The Longbottoms were never tortured to insanity by Bellatrix Lestrange.
James and Lily welcomed their first son into the world, Harry James Potter, two days after Neville was born. The two families got together often to talk, and occasionally the Weasley or the Diggorys, friends of the Potters, were invited over as well.
Sirius was never sent to Azkaban, and was named godfather. Harry grew up with his Uncles Moony, Wormtail, and Padfoot, along with his little sister, Aurora Lily Potter.
Remus fell in love with Tonks when Harry was six, and by the time he was eight, he had a cousin named Teddy.
Sirius eventually settled down with a Hufflepuff that had been in his year, and they had three children, two boys and a girl. Whenever they all got together at one house, be it for Christmas dinner or no reason at all, it was an automatic prank war. People left the house with neon flashing robes or hair, or occasionally spouting random words or songs.
"Come on Ron, Cedric, Neville ! We're gonna miss the train!" twelve year old Harry Potter shouted as he rushed towards the barrier, parents rushing along behind.
"Wait for me!" Aurora said, in front of her parents but still behind the boys.
"You know, mate, this is kinda your fault," Ron said, huffing as they heaved their luggage into the train. "You bet Teddy that he couldn't outdo his father in pranks."
"I didn't say it like that! I told him that he wasn't as good as his father, but he would be one day," Harry replied.
"Have fun at school, Prongslet!" Sirius called as the train sped away.
"Write home!" Lily called.
"Make Sni- Snape miserable for me! OW!" James shouted, then rubbed the back of his head where Lily had hit it.
"Goodbye, cubs!" Remus called.
"Bye bye," the various younger children echoed, along with some complaints of:
"But why can't I go?"
As parents and children waved their goodbyes and the train rushed off to Hogwarts once again, one thought floated in everyones' mind.
This is how it should be.