If there are no cameos made in the Fantastic Beasts movie by Harry Potter characters (from whatever generation) there will be no point and I will be thoroughly disappointed.

Okay but imagine this:

Regulus gets sorted into Gryffindor.

As he steps up to the stool, all he can think about is his parents’ scolding Sirius into oblivion, and how he can’t have that, not ever. But as the hat is placed on his head, he can see himself in the house of the lion, not caring about his parents opinion, and not having to worry. The word is said before he has a second to think: “GRYFFINDOR!” The hat shouts, followed by utter silence. For a good five seconds, no one moves, not even the Blacks themselves. And then the Gryffindor table erupts in cheers, and Regulus isn’t paralyzed in fear anymore as he takes off the hat and runs into his brother’s arms. Sirius is almost sobbing, Regulus is on the verge of hysterics, and James, Remus, and Peter are visibly tearing up. The Slytherins are, as expected, outraged that yet another has chosen to defy their ranks. Letters are sent home immediately, so word reaches the household quickly, making the next few days a living hell for the Black children.

Their friends try to help. Anytime a howler (noticeable by its folds) tries to make its way to their table, a lone spell manages to shoot it into a nearby trashcan. But the nonchalant looks of the Gryffindors suggest nothing happened. If a Slytherin tries to come over and attack the two, he/she is dragged into another hallway while someone else creates another route for them to take to wherever they need to be. But of course, it’s just because Remus found an interesting historical bit or something. At least once a week, a Slytherin can be found in detention for bullying, but it definitely wasn’t Peter who told a teacher about it. And when the Slytherins try to complain, Professors say they didn’t notice, but will keep a lookout for any bullying, adding in a suggestive glance towards the complainer.

Amidst all of this, Regulus is welcomed into the house like he’s been there all along. The Marauders let him into some of their meetings, and he even thinks of some pretty cool pranks (“He gets it from me, obviously.” “Shut up, Padfoot.”). Lily lets him study with her, and even offers to help him with some homework assignments (because defense against the dark arts mainly consisted of him getting scolded by the head of Slytherin, and therefore extra homework). Sirius manages to memorize both of their schedules, just in case, and so he can walk with Regulus as much as possible to reduce the amount of black (pun not intended) eyes per year.

By the end of the school year, James has already told his parents, and now the two Blacks have a place to stay if their house doesn’t feel like a home. This ends up in weekly visits from Sirius and Regulus, and they become full-time residents after nasty beating in Regulus’s third year (which is James and Sirius’s fifth). Eventually, the Potters host holiday meals for all of their friends, and the Tonks family are frequent visitors.

At the end of his seventh year, Sirius asks Regulus if he ever regretted choosing Gryffindor. His only reply is, “I think I would have regretted choosing Slytherin, it would have made Hogwarts a living nightmare.”

*laughs hysterically because that little rise in the 2000's is when the harry potter books came out*

How I think it happened

I’d like to imagine that the Marauders are all at James’s house when Sirius gets the letter. His owl soars in through an open window and drops the it on his lap. He picks the letter up and reads the front. The return address says “The Ministry of Magic.” Sirius cautiously opens the letter and reads it, then his face goes pale. He stands up and says he has to go, the other marauders give him confused looks but he leaves anyway.

Back at his house, Sirius goes into a fit of rage. He throws chairs all over the place, turns tables upside down, and messes up all the beds. He doesn’t care, it won’t be okay anymore. He screams and shouts things like, “WHY DIDN’T SOMEONE STOP HIM” and “HE WAS ONLY EIGHTEEN” only to lose his voice minutes later.

Then the marauders come in. Peter has let them in through a window because “You didn’t answer the door” and “we heard screaming” and they try to calm Sirius down because despite their efforts, he can’t be physically controlled. After easily fifteen minutes, Sirius stops, he just freezes. The marauders get a good look at him for the first time since he left Godric’s Hollow earlier that afternoon. He’s breathing heavily, his eyes are bloodshot, his cheeks are tear-stained and moist, and his hair (which he usually keeps perfectly clean) is a mess.

And then Sirius collapses. He crumbles to the floor, almost as if someones punched him in the gut, and cries. At first, the marauders are stunned, they don’t budge, and then Remus gets on his knees and hugs Sirius. Then, James and Peter join them, so it’s just them on the floor hugging Sirius while he cries. When they pull away, he looks even worse, and all he can say is just one phrase before he starts to cry again,

“I want my brother back.”