It just occured to me that the reason why Ryuken hates Quincies and doesn't want Ishida to cooperate with the shinigami is because his wife is being kept hostage by the Vandenreich.
In my opinion, the Ishida house is the most powerful clan amongst the Quincy with Ryuken and Uryu as war potentials that could wrestle control of the Quincy from Yhwach.
For years since the original war ended, Soken's clan had been keeping Yhwach at bay by using his influence and power among the Quincy to prevent the Vandenreich from gaining ground and starting another war. It probably mirroed the political climate of 20's Germany.
Once Soken became too old and passed down his powers to Ryuken, the latter had a small child and Yhwach took the chance to put the Ishidas out of the game.
By kidnaping Uryu's mother, they could persuade Ryuken to join them or at least not interfere as long as the war went on.
The conditions for keeping her alive would be that neither Ryuken or Uryu would oppose when the Vandenreich made their move and hence why Ishida didn't came to Hueco Mundo with Ichigo and the rest of the gang.
I think that, because of that non-agression pact, Ryuken didn't completely restore Uryu's powers during the Quincy training or at least deliberately imposed a limiter to keep him weak enough to be restrained and not represent a threat to the Vandenreich. This is why he appeared so lame in HM and Fullbringer arcs.
At the start of this arc, when Yhwach made his move on HM, Ryuken was informed the time to honor their agreement had come and thus he finally revealed the truth to Uryu so that the later would not jump on the battle and make the emperor kill his mother.
Knowing the Vandenreich, Ishida's mother would be killed at the first show of resistance from Ryuken, however, they are also savvy not to kill her before the war is over.
If they kill Ishida's mother, Ryuken will have no reason to hold back anymore and he will curbstomp a good deal of the Vandenreich so much so that Soul Society will gain a huge advantage.
Not to burst your theory, but we kinda already know a few things going against the war potentials being Ishida's, I'll focus on this aspect and not your theory at a whole, cause your theory as a whole is plausible.
If the Ishida's would be considered war potentials, then we need to know something Kubo hasn't told us yet, in my opinion, even if they're strong enough, which I agree they are, the plot just doesn't add up for it, yet at least.
I'll cover all my thoughts, also of those you also did think of, just to make sure you don't think I didn't read your post clearly, I did
Assuming the two unknown indeed are Ishida's, then what about all the other strong individuals? Why would it be the Ishida?
Don't get me wrong, I think they're helleah of strong too, but we're talking about war potentials who can be convinced to change sides, and not about all pure powers.
Yama-Jii was not a war potential, because he was loyal to the end of what he fought for, too stubborn, and uncontrollable, whereas he could come to an agreement with Ichigo, Aizen and Kenpachi, who all had the strengths he desired, and had the possibility to be convinced to join his side, whether he uses some trick to do so or not.
- Kenpachi in his old state just wasn't powerful enough, so he was cast aside, however he's now changed and got way stronger, what will happen next time he meets Bach?
Someone has to become a war potential, or it would be too stupid to even write this into the story, at least one of those 5 will have to bend to Bach.
- Aizen declined, simple as that, same as Bach knew Yama would, but he didn't know if Aizen would, so he had to ask him.
- Ichigo is still young, easily tricked, and impulsive, perhaps he had a chance with him, that's why I think he wants to return for Ichigo.
As for the last two war potentials, who would then be able to be convinced? The Ishida?
If they're holding hostages, which I think you might be right on, however there is a problem with them becoming war potentials with this, they would not fight for Vandenreich's goals and become one of them.
I believe hollows/arrancars are more easily forced, because that's their way of living, being forced by a stronger hollow/arrancar or die, they're pretty used to it.
When it comes to Shinigami and Quincy who are strong and always had free choice, then I think it's completely different, you cannot force them into ones goals, truly, when holding hostages.
If they're really holding hostages in order to get help from the Ishida's, then they're desperate, and then it would become a really bad story, cause the Vandenreich isn't strong enough on their own, and holds hostages to be able to win, instead of getting people on their side.
The difference is like night and day, an organisation desperately trying to find fighting powers by any means, or an organisation calm, strong and calculative, seeking out to get stronger steadily still with confidence in their own powers.
Desperate is not what I picture Vandenreich, however they might still hold hostages, but I think it's more of a means to stop the Ishida if needed be, and not to get them on their side, kinda like a reserve backup plan should it go sideways and the Ishida's join the battle.
Therefore I agree with you that they're holding down the Ishida family with something, this is also what I imagine, but by no means do I think they're considered war potentials, unless of course, they can manage to truly get them on their side.
They're not arrancars who fight more primarily by instinct, even if they got reason, after all.
Who knows, Ryuken may in fact be a war potential, but I think we know too little to make it a solid theory, it's still plausible, but I just want to stress these thoughts above
Just to short it all up, I don't disagree with your theory at whole, just the war potentials part I think we need more clarification on before we can name the last two unknowns.
The point of holding Ishida's mother hostage is not to force an alliance but to keep a powerful faction out of the conflict.
It's like if the Taliban captured prince Harry and demanded the United Kingdom to withdraw from Afghanistan or he dies.
As long as they have the prince, Britain would comply and remove their forces from the conflict. If they kill the prince, Britain will retaliate until all of the Taliban dies no matter what, however, if they release the prince there won't be any guarantees that Britain won't return to the conflict.
The only option for the kidnapper's faction is to keep the hostage alive indeterminately as long as the war goes on or they feel the balance of power has turned to their favour so that the hostage is no longer needed.
The focus of my theory is that the Ishida's have a huge deal of power which could turn the war unwinnable for the Vandenreich and for that reason they keep Uryu's mother hostage to prevent them from helping the shinigamis.
Either the Ishida gotta realize they won't save their family by ignoring the war, or it's not what keeps them out of the war.
I simply put don't want to see another rescue, it's gonna be a repeat therefore boring, so it has to be either of the two above in my opinion.
I don't think the Ishida are weak minded (kinda given in my opinion), they're not gonna sit and not being able to choose what to do.
I wanted to write way more than this as for my why I think different, but I'm out of time right now, however I want to stress that they're called War "Potentials", not something like War "Threats".
Potentials is constructive/positive word, which would be a bad pick of a word if he only wants to stop/kill/keep them out of the war.
Threat is more destructive/negative, Implying that these has to be stopped/killed/kept out of the war.
Hell Ichigo is even on the potential list, he went and asked Aizen as well, but seemed not to bother with Zaraki cause he was too weak, at least at the point.
Also as far as I'm concerned all translations say potentials and not some other word.
But as said I wanted to make a better post, but I don't have more time right now, peace
I like the idea that Uryuu's mother (or even grandmother) might be in the Vandenreich. There is that hooded Sternritter that I'm still curious about.
I also remember when ichigo was fighting Kirge, he was surprised to hear Ichigo mention that Uryuu was weaker then Kirge. So that let's me believe that they did possibly see him as a war potential but might have been mistaken... But Bach probably knows about Uryuu's potential and how fast he could grow in strength...
The back story just started, so you can't really assume anything yet about Uryuu's mother. For all we know, she could've died too, much like Masaki.
Regarding War Potentials, Kubo did confirm that two were Ichigo and Genryusai (who was later unqualified because of his missing arm) and Aizen (who refused, upon being asked). I don't think, Kenpachi was ever considered, being that their are more potential targets. Kenpachi has been overrated because of the whole "I forget my true self" thing and that, to me at least, don't make you the strongest.
Let's see, where the story goes further in the next couple of weeks. Katagiri may seem to build a relationship with Masaki, which would make more sense to the story plot so far.
I think their downfall will come because of Aizen, they took him into their HQ and that was their worst mistake.
When the next attack on SS happens, Aizen will stage a prison break with all Arrancars, Humans and Shinigamis trapped in the Vandenreich including Uryu's mother.
When all those prisioners escape, the Ishida's will make their move and Yhwach will be screwed.
Or, Juha Bach wanted disagreed with the "predecessors" and when they disagreed with him, there was a fight, a war whatever, and Juha bach was responsible for the death of them.- THUS- Masaki fled, or escaped Bach, by going to hide out and possibly marry into the Ishda house. I want to know what the holy power is. Is it like sklaver?
or some other super quincy technique,- Im thinking its Uber, and ichigo will be able to learn/ combine it for the final battle with bach.
However, the red herring is that Zaraki and Unohana fought in the same prison where Aizen was kept after his defeat, the place was empty so Aizen was no longer there because either Yuhabach kidnapped him or he broke free during the chaos of the invasion.
yeah maybe there was a Quincy civil war at one point?