You know, I hadn't thought about it until just now, but Bleach tends to mask it's characters abilities in some-might-say-clever ways, so that it's not that they just became more powerful, it's that they were already however powerful and you [the viewer] just hadn't realized it yet.
As a Vizard/Visored, I'd say Ichigo has just about reached the point where he can't get another MASSIVE upgrade without it becoming a farce.
HOWEVER
There's no reason he can't become more powerful through actual training in which he masters fine-control of his reiatsu or better yet, learns some freaking KIDO.
He's powerful in Bankai form, but if he trained in his Shikai form first, when he transformed, he'd be even stronger, from what I understand.
Of course, there's also the idea that Ichigo derives his power from or augments it through his emotional imperative to protect others to which is itself a direct result of him not being able to protect his mother.