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
War potentials
- Ichigo
- Aizen
- Kenpachi
- Unknown
- Unknown
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.