Do you think an imperfect hyogyoku (not fully awakened) creates an imperfect hybrid? Maybe it goes like this:
Ichigo is a perfect hybrid. He obtained his powers naturally.
The Vaizard group may/may not be perfect, we dont know how they obtained their powers.
We've seen that Ichigo can do a cero, ban kai, and can go full hollow with a hole in his body.
We've seen the Arrancar can do shikai's but we dont know if they can do ban kai. We also know they were created by an imperfect hogyoku.
We know a shinigami with ban kai after a couple of years is at the height of his abilities ie a perfect shinigami. (not to be confused with pure).
We know a Vastoorode is a hollow at the height of his abilities ie a perfect hollow (not a pure hollow).
My theory is that maybe an imperfect hogyoku breaks the barrier down in an imperfect manner. Most likely the amount of the barrier (between hollow and shinigami within a being) it breaks down is proportional to how awake it is. So if its 50% awake then we see a 50% hybrid. This would explain how we pretty much see Hollow Society. We see lower class arrancar with no zanpakutou. Then we see some with a general zanpakutou. Then we got some with a shikai.
So maybe (as stated before) a fully awakened hogyoku used an a hollow will create an arrancar that has the potential to reach ban kai and MAYBE he has a zanpakutou spirit that can teach him more about his ban kai and its abilities.
The reason i think Aizen hasn't used it on himself yet (if indeed he hasn't) is because he needs it fully awakened so that he may be a perfect hybrid and the path to Vastoorode will be available. What do you think?