Filler? I don't even know 'er! aka the fuck-Bleach [SPOILERS] megathread!

[Bells]

Ok, so... "Not-Hollow-Ichigo" fights Isshin, isshin gets severly wounded (very early in the series: you can drain someone`s reiatsu clean out of their body by wounding very specific points in their body).

Ok... how about this... that is not a Hollow, thats an Arrancar. created by Aizen. he fights isshin, wounds him severly (he looses his power) but gets killed. Due to his Hollowfication he infects Isshin, to an extent...

Eventually Ichigo is born (isshin is probably saved by ichigo's mom...) and that sliver of arrancar gets passed to Ichigo.

Arrancar are part Shinigami, Part Hollow, Ichigo if fully Quincy.

So he has a bit of Shinigami in him (which develops as Zangetsu) and a bit of hollow in him (which develops as Shirosaki) and his Quincy abilities allow him to achieve Fullbring a bit more easily AND explain why he had wild fluctuations of power... he is simply drawing tons of spirit particles all the time with little to no control. Also why he sees ghosts (and just one of his sisters)

So, rukia's meddling sent his Shinigami side to overload... and his hollow mask (which has slowly appear ever since the Rukia Rescue arc) also starts to grow at a slower rate.

When he gets in contact with more and more powerfull enemies, his Arrancar side starts showing up and his contact with Arrancar speeds things up even more.

Now, technically, he is free from his Shinigami Side and Arrancar Side, and his Shinigami powers are vestigial from the power his friends gave to him... so he must learn quincy techniques...

Which means... training arc with Ishida and Ryuuken.