The Hogyoku will likely be destroyed as a result of Ichigos FGT, in which cause I think Aizen will fall back to shinigami level but he'll be hurt, we prolly won't see his bankai yet.
Also Tatsuki stated before that she could feel an overwhelming reiatsu from Aizen, but nothing from Ichigo. In this chapter, Aizen explains that an evolved being can purposefully lower his reaitsu from 3D to 2D, which is what Aizen had been doing. Proof of that is later on in the fight Aizen explains that he's not even trying and he asserts if he so wished, he could "crush Ichigo with a single blow"
Also the issue with 2D an 3D is simple. Humans cannot see or sense Hollows, Shinigami, or + so we can surmise humans reside in the the 1st dimension. We were led to believe that there are only 2 dimensions that follow the same rules, but then we get introduced to the Kotoutsu. Gin describes that it cannot be destroyed by a shinigami as it exists in a different dimension. So we now have 3D's. I think.. it really has little to do with reaitsu levels, and more to do with potentials.
Just as Aizen pointed out that perhaps the true form of a zanpaktou is to be merged with its user, this is a potential for all shinigami just as reaching a higher existance is a potential for all beings.