I guess I'm not entirely understanding what Isshin's role is here and why he needs to be in a gigai. As far as I can tell, he's literally going to fuck the hollow out of Masaki.
For the Vizards, Urahara was able to concoct an injection that was a combination of Quincy light arrows and human souls. These, as the natural opposites of what the Vizards were (Shinigami and hollows), provided balance and allowed them to not go through soul death. As a mere injection, though, it couldn't actually push back their hollowfication, just keep them from dying.
The goal with Masaki is to actually halt the hollowfication. So they need a more potent version of that same idea. Masaki is currently Quincy/Hollow, so her balancing factor is Shinigami/Human. Isshin is only one of those things, but he can become the full equation if he goes in a soul-suppressing gigai. Ryuuken is already quincy, there's nothing he could even combine with to make the two things he'd need to be. In theory Urahara could do it, suppress himself in that Gigai forever, but "ha ha no thanks."
The idea is to create her opposite--a Shinigami/human--and then tie it to Masaki with reishi strings so it's a constant balancing factor. That tying basically being the equivalent of the injection the Vizards got, but much more potent. They'd have to be tied until the day she died, in order to keep the hollowfication in check.
