This is a question that I've been pondering since Yamamoto died and the Fullbringers appeared in Soul Society. It has been stated in the Manga that when a soul from Soul Society dies, it returns to the world of the living, this keeping a balance on the number of souls. If this is the case, are all of the Shinigami (including Yamamoto)'s souls back in the world of the living? Another question to ask, if Quincys have the power to obliterate a hollow from existence, does that mean that they can also obliterate other types of Souls (such as Shinigami, Humans or other Quincys)?
Why am I asking these questions? Well, I've been really meditating why it is that the Fullbringers were able to appear in Rungokai and be picked up by Ganju Shiba. The Fullbringers are ultimately human who died in their human bodies and were sent to Soul Society, it would explain why we are able to see what happened to them (similar to what happened to Yūichi Shibata). However, it has been a mystery why Masaki has never appeared in Rungokai (and of course, one could argue that Rungokai is huge but for Soul Society's 12th Division, locating a soul shouldn't be difficult). However, in the case of Yamamoto and all of the other Shinigamis that were killed by Quincy, my personal opinion and theory is that it means that their souls no longer exist in any plane of existence. If this is shown to be true, then it would explain who is responsible for Masaki's death and it would explain why the Quincy are more terrifying and a serious threat compared to in the past.
One last question is if Shinigami (as well as Quincy) have the power to obliterate souls like the Quincy (are some Shinigami able to do this or do all Shinigami possess this ability)? Would love to hear what you think about this.
I would love for all of you to state your opinions (no speculations here) on what you think about this theory. :)