Grimmjow already pretty much said how he couldn't become a Vastolorde because he was at his limit. I would like him to significant again too but unless his contract with Urahara is to remove that limit I don't see it happening.
Maybe that is his contract with Urahara though. He agrees to help them with something that only an arrancar can do and Urahara agrees to help break more barriers or some such nonsense.

