It's simple Logic:
1) Guy invents Zanpakuto's...Guy knows the inner workings of Zanpakutos.
2) Guy examins unsealable Zanpakuto
3) Guy notices X-Factor that separates unsealable Zanpakuto from all other Zanpakutos
This guy ought t be tossed out of Soul King's Dimension, if he can't discover how Ichigo's Zanpakuto is different from the other Zanpakutos in SS, and from there reverse engineer a way to prevent sealing or modify other Zanpakutos to make them resistant to sealing. It's just plain logic.
Now, does kubo wish to go this way or not? That is the only question.
Ichigo's Quincy heritage is the reason why he was able to escape Opie's Jail, and probably why the Quincy's bankai-stealing medallion doesn't seem to work on him.
The next zero aquad member should(plot-wise I mean) only be able to somehow fix, and even upgrade, both Ichigo's and Renji's bankai, nothing more, nothing less. That alone would be a testimony to his proper place within the zero squad, considering the mad-scientist captain, is not able to do such things.
Any thoughts on the historical impact the other two zero squad members have had on soul society? The first three have been rather diverse thus far; mod soul, healing springs, and the soul cutter.
Bach forbade other Quincy from staling Yamamoto's bankai (which means that they would be able to), because they wouldn't be able to fully utilize it, the same way as the Spear Guy was unable to use that Thunder Bankai to it's fullest. In similar manner, As Nodt would probably be unable to invoke the other stages of Senbonzakura Kageyoshi.
Now that I think about it, shouldn't they have accepted Yoruichi into the Squad Zero ? She created a new technique/fighting style, by combining Kidou with Hand to Hand combat (what was it called again ?). This seems like a discovery on kidou or Zanpakutou's level, it simply wasn't introduced through out the SS yet.
Yoruichi should be a Squad member you say? Hmm... Each of them holds the power equivalent to 2 & 1/2 captains or 2 captains and 1 VC. Each of them were like an Issac Newton or Einstein bringing into fruition some drastic uber hax change elite enough to make history! Where does Yoruichi even BEGIN to compare to that?
Yeah she made yammi look like shit (who doesn't?) and Ulq was wise enough retreat acknowledging her strength. (Not to mention Urahara was there). Yes she designed a technique with the already existing Kidou (there's plenty of that).... She doesn't get anymore badass than that. Had she been the mother of Kidou you'd have something to debate with.
^ If that was the case, then almost no Quincy would have been able to steal a bankai, since Quinces have a low amount of power themselves. Their inner power levels are much, much weaker than those of VC and Captains, since their strength lies in absorbing and controlling the energy around them. The only time when their power levels might go up, is when they enter into their Complete Holy Stage, where they fuse with the reishi that they have gathered from all over the place.
Their Blood arts are so effective, because they condense the reishi from atmosphere and run it through their veins and arteries, that's why they can surpass the damage dealt and defense of Shinigami. This would mean that in an environment with low amount of spirit particles, their techniques would become weaker. It is a common trait of almost all of Quincy abilities.
Their Bankai-Stealing-Medallions is like an extension of their natural ability to gather and control reishi, where they simply take away the core power of their Zans and put it into a medallion. The reason why they can steal it in the first place, is because to unleash a bankai, Shnigamis need to put their full Core of their Zanpakutou into their swords. That's why it's impossible to heal a have damage done to bankai and that's why it's easy to steal it, since it is already partially separated from the user.
I think that they can steal bankai of people much stronger than them, but the pressure created by the bankai itself might be lethal to them. In similar manner, Ichigo was crushed by his own Bankai's spirit pressure in the bankai fight against Byakuya. The Mustache VC's example simply shows that then they release a weaker, or even a much weaker version of their bankai. Heck, it was even weaker than the very first version, without any improvements, of the Mustache VC's bankai, since in the flashback, he presents his new bankai to Yamamto, which is stronger than than the one used by the Spear Quincy (not to mention that it was improved after that).