Rocky you forgot the whole bit about ichigo's heart disease, and the son of renji/orihime is traveling back in time to save ichigo.
Btw naruto is pretty much in the same boat atm, his last powerup made him a ****ing god, the only pacing mechanism being that he sprained his ankle. lulwhut? (wanted to put this at end of post, but noone would read it there

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In seriousness though, there are some plot elements that indeed could've enabled this "powerup". But first of all my main gripe is just how overhyped aizen was. Sure it makes ichigo look 110% more badass now, but it also really annoying. I mean what, there were like 5 chapters where all we saw was aizen chasing puny humans, obliterating gin and pulverising everything around him, with him becoming more and more douchebag.
I mean the whole thing wouldn't be so ridiculous if aizen wasn't shown as Jesus himself riding on a velociraptor unicorn that is on fire shooting a gun that uses shuriken and lightning as it's ammunition. All accompanied by a choir of newborn angels with electric guitars and the most badass drum solo since the battle of jericho.
I mean it would've been sufficient if aizen beat everyone else simply because they were tired from fighting the top echolon espada + tousen. But boosting his power to the level it was done was kind of too OTT.
Anyways, back to ichigo and the plot elements that can make a powerup like that possible:
He's a human with shinigami powers. Apart from his father I can't think of anyone else with those characteristics, and it still has to be shown that his father is not a shinigami in a gigai, which would make ichigo a shinigami/human hybrid.
He also has a hollow inside him, so he's a human with shinigami and hollow powers.
More importantly, and I said this in a post some time ago, All of his power evolution so far has been forcefully pushed out of him in some way. His shinigami powers were forced out by urahara, awaking his hollow at the same time. He never controlled his shikai, leaving it to be always in release. When fighting kenpachi, he forced even more power out of his zanpaktou. His bankai was forced by the training doll, which also led to the suppresion of "old man" and the hollow taking over his inner world. His vaizard transformation was forced out by defeating that inner hollow.
His vaizard v2 (or full hollow form) was forced out by orihime crying about his death against urahara. I believe this form to be the polar opposite to his current form, whereas now he is at the pinnacle of shinigami evolution, while the full hollow form is his reiatsu set loose.
What I'm trying to say is, that all of his powerups have been completed in fractions of the actually required time and were always accompanied by force. Usually for shinigami acquiring bankai involves strengthening the bond between you and your sword, and even once you have achieved bankai, it takes centuries to master it. The vaizard had 100 years to develop their hybrid powers.
What I'm trying to say is that during these 3 months, for a first time Ichigo went into himself to talk to his zanpaktou. That might be the reason why he could tap into powers that seemed unachievable before.
I still very much think that bleach is awesome, just that tite kubo overdid it with portraying aizens power, which led to the ridiculous situation we are in now (and I really hate that). But even that can be taken with some level of explanation. As said, everyone that fought aizen was already low on stamina from battling before (apart from urahara and yoruichi, but these have been in exile for 100 years). Everything after that was aizen v puny little humans.
Exception being aizen being all "no u" when gin killed him.
Anyway I do realize that this post if fricking long, so kudos to anyone that bothers to read it.