While it's all hypothetical at the moment, a recent interview with Kubo hinted towards the possible return of Grimmjow and Nel as friendly [allies]. You can read about it here on a journal entry from our deviantart group listing:
http://GrimmNel-club.deviantart.com/journal/KUBO-SAYS-NEL-amp-GRIMMJOW-MIGHT-RETURN-281453288 Even if it's a remote possibility, the thought is rather exciting, isn't it? Here's to hoping that Kubo decides to follow through with this.