When Ichigo lost his powers due to Byakuya stabbing him in a critical area, Ichigo went to Urahara for help in regaining his powers. What Urahara did was separate Ichigo's soul from his body as well as sever the chain that connects the two. As it is explained in earlier episodes, once the chain is severed, a plus deteriorates and gradually and will eventually become a Hollow.
Right before Ichigo succeeded in reobtaining his Shinigami powers, his exterior took the form of a massive Hollow. He managed to shatter his own mask and come out a Shinigami.
But wait, there's more.
Possible exposure to the Hogyoku via Rukia as well as Ichigo's own naturally high reiatsu could have inevitably result in his transformation as a Vizard. Urahara knows a lot more than he's showing. What peaked Aizen's interest in Ichigo before he found him useless was the fact that Ichigo is in some ways both an Arrancar and a Vizard.
The normal requirements to be an Arrancar is for a Hollow to shatter his own mask or get help from an outside source to evolve even further and gain more Shinigami powers. This outside source was the Hogyoku.
The requirements to be a Vizard is unknown, but it is possible that the Vizard gained their powers thanks to Urahara's help. Urahara probably used the Hogyoku on his former comrades as some sort of freak accident or perhaps some other cataclysmic event happened 108(?) years ago that resulted in the Vizard becoming the way they are.
What is Ichigo? If you recall his transformation back into a Shinigami, his body took the form of a Hollow while his subconscious was searching for the means to become a Shinigami. Once he found that, he had to break free from his proverbial shell and the end result was his Hollow mask shattering.
The requirement to be an Arrancar is fulfilled. The requirement to be a Vizard is fulfilled as well once Ichigo gains the power to control his inner Hollow and borrow power from him. That is what makes Ichigo unique from all the other Arrancar and the Vizard. Both personalities are intact whereas the Arrancar is just usurping power that resembles a Shinigami and the Vizard is "enslaving" their inner hollows and bending them to follow their will. Ichigo's inner hollow is no slave; rather, he is watching from the sidelines, masquerading as the spirit of Zangetsu.
Theoretically, it is possible that Ichigo could go Arrancar if he gave in to his inner Hollow and instead of the Shinigami borrowing powers from the Hollow, it would essentially be the Hollow borrowing powers from the Shinigami.
If Ogihci (the "name" of the inner hollow as specified from one of the cover pages) were to gain control of Ichigo, he would have his own Ressurrecion release like the rest of the Espada and Arrancar.
That could possibly be Ichigo's next powerup.