some Bleach 4th level fan theory stuff..
! Ok here is a theory, not sure its a crack theory because i have quite a few points that accredit my theory imo but here it goes.
! JUHA BACH IS THE ORIGINAL SOUL KING. Ichigos relationship to juha bach; we know that ichigo has quincy blood and his mother was a quincy. Also juha referred to ichigo as "My son", and wanted to "reeducate" him. Also Haschwald looked pretty upset when juha was beating up ichigo. This leads me to believe that YES ichigo is somehow indeed related to Juha Bach (either as a son or a descendant) and that ichigo has some juha bach in him too. This is solidified by the fact that ichigos FGT form and Zangetsu look like juha. There is indeed a resemblance! So yes people Juha Bach's power does lie deep within ichigo. So its safe to assume that the FGT form is derived from juha bachs power.
! Next part; The Hogyoku has part of the soul kings power. Remember Aizens evolution forms. He went through the chrysalis stage and became a butterfly. The soul king awoke from his slumber (just like a butterfly) and his eyes resemble those of aizen. Coincidence, i think not. The hogyoku was created by urahara. Recently in the manga it has been stated that urahara has been to the royal palace in the past and did some research. So urahara probably analyzed the soul king's power and tried to recreate it. If the hogyoku senses the hearts of those around it and materializes their deepest desire, the soul king can do the same thing. In fact the soul king probably uses this power to keep the balance of soul, basically do his job lol. In order for aizen to kill the soul king he had to be on the same dimension! So he acquired part of his power from the hogyoku, and therefore becoming an ascended being, making him confident and capable enough to kill the soul king.
! With all that being said heres where everything comes together. Ichigo in his FGT form was standing on a higher dimension then butterfly form aizen or "soul king form", that same form that would make aizen capable of killing the soul king. See where im going? Does that mean that the power of juha bach is standing on a higher level then the power of the soul king? But there is no level higher then the soul king himself, its impossible! if thats the case then who is juha bach? ?????
! For my previous 3 theory's to make sense and for EVERYTHING to be connected, there can only be one answer to this question…..
Juha Bach is the former soul king!!!!
! Yammamoto founded his own academy of shinigami 2000 years ago. During that time Juha and his quincy clan where in power and juha was the soul king, ruling over the world, which explains why everyone is calling him his majesty. But his rule was evil as the world was slowly going on the verge of destruction because of quincy powers crumbling the balance of souls. Yammamoto never acknowledged Juha and decided to rebel against his rule. 1000 years ago, Yammamoto along with his subordinates decided to raid the soul palace and overthrow juha bach. They were merciless and murdered all the Quincies. Just like terrorists trying to overthrow a government. Yammamoto and boys succeeded, and the soul society was formed. Although he never acknowledged him as the soul king, Yammamoto respected juha bach and decided not to kill him, and banished him.(Into the shadow realm?) Now Juha Bach is back to reclaim his throne, restore the former glory of his clan and take down his successor.
! Theres probably alot of loose ends, but we know the Juha Bach needs to be powerful enough to take down squad zero and the soul king.
! This theory is credited to bleach1234567777 on youtube. such a great theory i had to forward it to the forums.
! Intresting, I have red a lot of Japanese Bleachers have very simmilar theories. Also unlike many forigen Bleachers they seam to take Juha Bach's "My Son" statment mutch more serius then the rest of us. (I have barly seen anyone who think it's just methaphorical speaking!)
I see nothing that goes aganst this theory of his and I think it's quite possible that Kubo might do somthing like this 
I have been thinking somthing like this theory as well. I am still going for the theory of Ichigo being somwhat not Juha Bach's biological son but rather is "spirit" son in a sense.
I remember in Ichigo's fight against Jackie, the symbol Ichigo where trowin at Jackie is the same shape as the Quincy medallions
(See my signature pic)
And Bach does resemble Zangetsu a lot, somthing I have a feeling being more than kubo's recycling designs… Plus his sword has a simmilar desing to Ichigo's new Shikai sword
Him beeing the former Soul King woud certanly make things very intresting in my eyes! x3
! Japanese BLEACHers take the "my son" line serious, cos… lol ...they all recognize the relationship being setup.
! The most direct reference would be, the Hecatonchires from Greek mythology, who were sons of Chronos (the God of the Material World, who challenged/overthrew his father, the God of Heaven) said to be more powerful than Chronos himself! (quick note; their names mean: "100 handed", "vigorous", "furious", and "big-limbed"... sound familiar?).
! What's being setup here is essentially, the story whereby the "Son of Man" (i.e., a mortal) takes on the mind/power of the "God of Heaven" (thus becoming the "Son of God"), and rebels against the powers of the mortal world (represented by Ywachi aka YHWH aka Chronos aka Saturn/Satan).
! [Note that Chronos/YHWH is the material world, his is the LOGOS "I exist, in order that I continue existing" (aka "I AM; THAT I AM"); and everything existing under/within him has this as a fundamental part of their existence. Creatures outside of the material world take their existence for granted, hence their somewhat "uncaring" presentation.]
! Back to the point, this is then where the ambiguity regarding Ichigo comes into play: this "Son of Man" who is also the "Son of God" can rightfully be claimed by either Chronos or Ouranos: he (Ichigo) is the direct offspring of Chronos, who is the womb/"matrix" through which Ouranos chose to create/craft him (Ichigo). Within the story world, that'd probably make Ichigo's mom Ywachi's daughter.
! Now, where does Isshin fit in? Isshin is the 1st "Son of God", whose advent was necessary in order to create the ideal one, Ichigo. Proof is in the name of is Zanpakutou: ENGETSU ("moon incinerator", vs. Ichigo's "moon cutter"). They are both ALPHA & OMEGA, CREATOR & DESTROYER, but Isshin likely abandoned his duties (as DESTROYER) for the sake of woman (hence his exile, hiding, and the downfall of the Shiba clan). This is something that biblical sources refer to in numerous places as well.