WMG / No Game No Life

Sora and Shiro are reincarnated Old Deus.

Specifically, they used to be the Anthropomorphic Personifications of Chaos and Order. Sora is Chaos because he is passionate, charismatic and he's better at Dating Sim then Shiro which is based on emotion and reading people. Shiro is Order because she is systematic, heavily focused on rules, and a master of strategy games like Chess while she emotes little and overall has a quieter presence than Sora.

This would explain their shared identity of "『　　』"

1. Chaos+Order balances out to nothing or otherwise "blank".

2. "『　　』" never losses because two of them together are effectively a Yin-Yang Bomb

3. They can't stand to be separated because they truly are a shared existence.

When Tet said they were "born in the wrong world" (s)he wasn't speaking hypothetically. They were supposed to be reborn in Dishboard but strayed over to Earth. Jibril laughed at the idea of permanent summoning from another world because even Old Deus would have trouble with that, but if three Old Deus were involved they could use their own power to maintain their existence. Tet brought them back so (s)he could have a game version of the Old Deus war.

• In Volume 6, its revealed that Sora and Shiro are the reincarnations of Riku and Shuvi, an Imanity and Ex-Machina who existed 6000 years ago.

Sora and Shiro will collect all sixteen Race Pieces during their conquest of Disboard.

• Tet's own piece - the Old Deus Race Piece - is, in all likelihood, the black king, just as humanity's is the white king.
• What with them being angels, it would make sense for the Flugel to have one of the four bishops.

The name "Tet" is possibly inspired from "Tetris", and there should be also "Ris" to make a complete Tetris.

After all, Tetris is a classic game. It is also possible that Tet is playing Tetris during the Deus war. Ya see, he is the God of game, and since no one would need a god of game during war, he got very bored and thus played Tetris till the war ends.

The old king lost the library to Jibril on purpose.

Obfuscating Stupidity aside, this would put him into the Crazy-Prepared category. Having a high ranking member of the Flugel on Imanity's side would certainly be a boon. A Living Weapon is living in the national library and is thus accessible to his successor. He's banking on the fact that the person who figures out his clues on how to trounce the Warbeasts has enough brains and balls to battle a Flugel.

• He also probably expected Steph to give his successor the key unconditionally, which would have sped up the proceedings. Still, it's not like 『　　』 Sequence Breaking had any actual negative effects on the plot. Having a Lightning Bruiser Magic Knight on their side wasn't strictly necessary to challenge the Warbeasts, especially since their games are unaffected by magic, but it did help. And since the Warbeasts are canonically the only race with video game technology, Jibril ought to be at peak performance in every fight after this.

Tet will deal a major blow, if not win the first round, against "『　　』"

A major theme of Blank's wins is that the opponent underestimates Blank or Imanity as a whole and that leads to their defeat. Tet obviously doesn't have this problem. He knows full well what both Imanity and Blank are capable of, and he's been watching their every move since they arrived on Disboard. Which plays into another theme: Information and Preparation. Tet sees everything Sora and Shiro can do, both separately and together, but aside from the chess game Sora and Shiro have no intel on Tet, unless the games he played with Izuna are going to become a plot point. So it only makes sense that he's gonna have a secret plan for whatever game Sora and Shiro throw at him that will take them aback and put them on the defensive.

The whole story is Sora and Shiro's Dying Dream.

Because what's more likely here— that a whole fantasy world based entirely around games exists and can create a portal to the real world, or that two orphans who literally spend every minute of every day playing video games and are unknown to the rest of the world die of starvation once they inevitably run out of food and money, and have a shared hallucination? Depressing, yes, but it's not the first time something like this has happened.

• "This unrealistic fictional story must be a dream because none of this stuff can exist in real life!" Sounds very stupid and there is no In-Universe evidence to suggest that these two fictional characters are in a dream. If everything in fiction has to be realistic, do you believe that Dying Dream's are real as well? Also, just because a character was revealed to be in a Dying Dream in one other completely unrelated story, doesn't mean that it has to happen Every. Single. Time. Imagine if every story ended the exact same way. Every, single, time.

Alternately, Sora and Shiro were actually pretty much set for life and didn't need to work.

Shiro in particular blew away pretty much every intelligence test before she was three. They would only really need to capitalize off of that fame to get a pretty decent trust fund to begin with. Add on top of that things like stream money [I can imagine Shiro doing supposedly tool-assisted speedruns without tools], prize money from online tournaments, prize money from online poker games, and whatever money that their parents would have left them in the first place, and it's not hard to imagine them doing pretty well.

Also, there's no way that Sora would ever let the money run out. He has to pay the internet bill, because he has sexual needs, sexual needs, sexual needs, and even sexual needs.

Marry Them All will be featured in the future.

Sora and Shiro will end up married (or otherwise bound) to someone from each race, securing the races for quite a while—likely taking one of each race from among their followers. Probably as follows, based on Dating Sim tropes:

Imanity: Steph. Duh.

Flugels: Jibril. Mildly Tsundere and Yandere.

Elves: Feel, since she's so far the only elf who isn't terrible. It could be a political alliance thing to defeat whoever is in charge of the elves.

Warbeasts: Izuna. 『 』 seems to like dog girls, given that they dressed up Steph like a dog that one time, and they've already got a thing going with her.

Damphires: Plum. He's the Otokonoko option.

Sirens: Leila, but only because of her and Sora's kid.

Sora and Shiro will return to Earth at the end of the series.

In one of the lns, Tet says that the duo could never compare to Riku and Shuvi because the siblings ran away from their problems while the couple faced theirs head on. Once Blank reaches godhood, what would be next? By that point they would have already beaten the rest of the Exceeds, which are represented by their best players. The only people they would have left to beat are their past lives in the one game they gave up on: Earth.

Naturally, Jibril will follow them; why should she let something petty like dimensional distance keep her away from her masters? Plum will come too (or be dragged along by Jibril) because of a G-Rated Drug addiction to Sora's bodily fluids. Perhaps some others as well. After all, if Tet can bring the siblings to Dishboard without trouble, there's no reason (s)he can't send others to Earth.

Instead of the Ten Pledges of Tet, the new game will follow the Six Pledges of Riku, which means their greatest challenge will be stopping Jibril from vaporizing people. Seriously, after the inviolable rules and Magically Binding Contracts of Dishboard, what could be a better challenge than operating under restrictions that their opponents will not have?

• That is, assuming Tet is allowed to do that. Earth might not be a viable place to play as it doesn't mean that the rules of Disboard applies to other worlds.