Excerpt from OOTP:

Only one pair was still battling, apparently unaware of the new arrival. Harry saw Sirius duck Bellatrix's jet of red light: he was laughing at her.

'Come on, you can do better than that!' he yelled, his voice echoing around the cavernous room.

The second jet of light hit him squarely on the chest.

The laughter had not quite died from his face, but his eyes widened in shock.

It seemed to take Sirius an age to fall: his body curved in a graceful arc as he sank backwards through the ragged veil hanging from the arch.

Harry saw the look of mingled fear and surprise on his godfather's wasted, once-handsome face as he fell through the ancient doorway and disappeared behind the veil, which fluttered for a moment as though in a high wind, then fell back into place.

Sirius did not reappear…

Sirius Black, Marauder and Dogfather, felt true regret as all light disappeared as he entered the Veil of Death.

He had no regret of dying, and in fact felt a little bit of relief. He had been forced to stay in his horrible family home and any reason to leave was a welcome one. That he had rushed to save the only light in his current existence should surprise no one.

He had no regret over the nature of the incident which pushed him into the Veil. If he couldn't die in bed with two hot Swedish-twin witches, then fighting the good fight was a reasonably acceptable way to go.

No. Sirius' true regret was that he would be barred from spending any more time with his well-loved godson. He could only hope that his death bought a distraction and time for Harry to be saved or to save himself – preferably the latter.

Having made up his mind to accept the consequences of his actions, Sirius put all such conjecture out of his mind and attempted to figure out where he was or what was happening. There was still nothing to be seen, but as he concentrated, he could make out distant sounds.

"… not his time …"

" … not our place to choose the … soul … earned …."

"… do not accept … cannot … ."

The voices became clearer, as if they were moving closer.

"As Keeper of the Way, it is my place to send him to reward or punishment. His soul is whole. He has not damaged his own soul or the soul of another."

"But he is not innocent. He cannot be sent to taint those who have been sent to where the pure souls go."

Sirius tried to talk. He wanted to tell them to send him to wherever Prongs and Lily-Flower had been sent, as he knew that they would be watching Harry. He could watch Harry too. And maybe give them comfort if they were worried.

But his voice would not work.

"Look. Look at his soul. He has accepted responsibility for his past. Even now he thinks of how he could serve other souls. He does not deserve to be sent where impure souls go."

"You are the Keeper. Your choices are final. He cannot go to where the pure are sent. He has not earned going to where the impure are sent. He cannot be returned to the time and place he came from, as this would invalidate the Edict by which we serve. What will you do?"

There was a long pause. Finally the first voice, the Keeper, spoke as if he was thinking while talking.

"No. You are right. He cannot be sent back."

"But he cannot be sent for oneness with the Ultimate or final destruction. He should be sent for another turn on the Wheel."

"But the Edict says I must leave him with his memories, his knowledge, so that he can appreciate why he has been sent to the appointed place. In the past, I have only received the guilty which could be punished or the martyrs who could be cleansed. This one was sent by chance. If I send him another turn on the Wheel, resurrected in a new body, he will still remember his identity as it was before he came to me. The old memories in a new body might damage a newly-born body."

"But there is no surety."

"True. But look: His thread remains. He cannot be given a new one until the old one is cut or destroyed. It is damaged. It untwined from the weave but was frayed because it was attached strongly. He was well-loved by at least two. But there is a portion left of his thread in the former weave. It will provide a nebulous comfort to those left behind. But his thread even now seeks a new weave to attach to."

There was a much longer pause as the two voices seemed to contemplate. Sirius had given up on trying to speak – nothing he did could distract the keeper or his companion. And so he waited with both anticipation and dread.

"Consider. Does the Edict only give me these choices: Punishment, Reward, or a new life? Am I forced to choose only of these three choices?"

There was another pause and the send voice sounded surprised. "No. You have complete choice and your decision is final and absolute. You cannot be gainsaid. You are the Keeper of the Way, and you are directed to send any who come to your court to the proper destination, according to the nature of the soul which comes. But in no way is the proper destination defined."

"If a new body might be damaged by his memories, might he keep his current body?"

There was a startled pause. "This has never been done."

"But does it violate the Edict?"

"No. The current body is wasted, though. In the normal course, it is my place to detach the body so that you can direct the soul. That is why I am your companion. Your choices are supreme here, as defined by the Edict. But as it often is when one comes to the Way, the body has suffered. Would it not be a benison to the soul, to relieve it from suffering the pains of the infirm body?"

There was a pause. "This is true. If I give you leave to perform your normal offices, I could send the soul to another body. I could chance a newborn. But I might send it to one already living."

"You must consider that this will result in chance. If this soul enters a body of a soul with a strong thread, the threads will merge. If the soul present is weakly attached, it will be sent on and this thread will supplant the other as it attaches to the weave."

"But perhaps you can heal this body rather than destroy it. You do have the power and the ability."

There was another pause. "Yes. This is true. All choices you might make will affect one weave or another."

"What effect could be made to a weave?"

"A newborn body will allow it a fresh connection and will provide the least disruption. Sending to an existing body will change a weave more subtly. But sending this body with its current thread will affect a weave most strikingly: The thread will attach unpredictably. Do you wish to chance disrupting a weave so thoroughly?"

There was a chuckle. "But we both know that many weaves are already teetering toward destruction or move toward chaos already. A random thread attached strongly might stabilize an otherwise undesirable or unstable weave."

"It is as you say." There was a long pause. "Save for final reward or final destruction, any choice is open to you. Choose. And I will support you in your choice, as is my place."

"You have my thanks." There was another pause. "I have made a choice."

"And the choice?"

"We will …."

Such is the beginning of this collection of stories.

Sirius could end up in any universe or any dimension. He could be in another timeline similar to his original, or any extreme variant. He could be merged with an existing character or replace one. He could end up in any universe.

There are over 850,000 Harry Potter stories and crossovers. And there are many tropes that exist, overplayed. I've even started a few of them which are almost canon fanon now.

Let's see if we can branch out from the normal stories.

As an aside:

Nowhere in the book is the spell Bellatrix sent defined. Stupify is red, as is Crucio. But neither matches up to the effect.

In some fanfictions, she used a blood-freezing curse. And that is consistent with her character.

But the book describes no trauma other than fear and surprise. Fear could be because the Veil is there. Surprise could be because the spell used was not one Bellatrix would normally use.

The only result from the book is Sirius falling in an arc, which implies a physical force. It can't be the knockback jinx as this is blue. But a similar effect seems to be created.