I'm going with a theory that the swords can have there own family based thing going on.
For those who are interested
Theory] Zangestu, Engetsu, and Kyoka Suigetsu
so this is all about opinion mostly about the zanpaktous of isshin, ichigo, and Aizen... and not only that but their connection to each other...
Isshin being father of ichigo...(hopefully).. has the 'getsu"same as Ichigo and Aizen * hopefully everyone notices...
The point that annoys me is that if Aizen is related... or has a connection... why wasn't their any sign when out of the blue isshin came and told Aizen "you talk to much".... even thought saying that line means.. he knows him very well... Doesn't mean that Aizen knows him.. * hint hint*
Due to that... I think that real world relation has nothing to do with this... but zanpaktou world do...like isshin said.. they are that "type".. leading to my theory that. it is a crazy one...
Every Zanpaktou has a 'family' and in each of the member of the family zanpaktou have different characteristics and powers...
like music has a family in an orchestra.. and in the family different instruments have a different purpose..
what I am trying to say is that...for example renji... zabimaru... may not be "that type of zanpaktou" where they fight to teach/ learn... w.e
Overall.. I think that there is no relation between Aizen, Isshin, Ichigo... but with the zanpaktou...
your opinions on the relationships???
One more thing:
I remember people talking about isshin zanpaktou when we first seen him enter, and there was alot of comparasion that isshin zanpak, is similar to aizen and ichigo... this could also be a link to the ' family zanpaktou'
Now hes abit of something I believe only two that are closely related seeing as both Engestu and Zangetsu mean crescent moon, while suigestu is watermoon so Azians newest sword name is like a very distinct relative to the other two
(BIG SPOILER COMING UP I GOT TOLD AND TRUE BUTWITH MY THEORY AS TO WHY Isshin lost his powers aswell]
The Final Getsuga Tensho is, in fact, functionally identical to the Quincy Final Form. It is an attack that is far above anything Ichigo or Isshin could ever possibly use normally, capable of killing anything it hits, but that also sacrifices there Shinigami powers. This could be used to explain how Isshin lost his powers 20 years ago, if he wasn't exiled.
It didn't happen in the actual anime yet...you have to read the manga...see chps. 425-429 when he learns the final Gatsuga Tensho...if ur talkin about when he learns bankai i have completely wat episode tht was.