Ok, so I believe most Grimmjow and GrimmIchi fans are kind of disappointed because Kubo didn’t grant our wish, but there will be a flashback -there must be- of what happened in Hueco Mundo and how the hell Ichigo got free of that cage.
Now, if the mysterious swordman turns out to be Grimmjow, which I’m 90% sure he is, then he somehow manages to destroy the cage and liberate Ichigo. Of course, Ichigo is completely perplex knowing that the guy who wanted to kill him so bad is now his saviour. And Grimmjow says something like “That’s for the time you saved me from Nnoitra, now we’re even, so do whatever you have to do and when you’re done I’ll kick your ass!”. But even though Ichigo is grateful to Grimmjow, he is still worried about his friends. Urahara says they will be fine,they’ll go rescure Harribel and that he should leave as soon as possible, and Grimmjow tells him that he won’t do anything to them since they are not his problem.
At this point, I made myself sad because I really want to see Ichigo and Grimmjow fighting together. However, I don’t think Grimmjow will go to Soul Society with him to fight the Quincies. Why? Because he has nothing to do there. He only wants Hueco Mundo back to its normal state, he doesn’t give a single fuck of what happens to Soul Society, so why fight there? He is only concerned about the weirdos with arrows who are invading his territory and fucking everything up. It’s his mission to defend Hueco Mundo, just as it’s Ichigo’s to protect Soul Society.
But then, if Orihime, Nel, Chad and Urahara stay in Hueco Mundo, he will help them, explain to them what has been happening there and then they will go rescue Harribel. But we know Grimmjow is someone who prefers to do things on his own and it’s not like he would lift a finger for a fellow Arrancar if they were attacked. But that’s what would happen in normal circumstances. Now there’s an opponent way stronger than him (and I don’t doubt there are some Sternritters left in Hueco Mundo) and he is clever enough to admit that he can’t beat them all by himself. Moreover, most of the Arrancars have been either assassinated or recruited by the Quincies. Therefore, what he plans to do is liberate Harribel and the other Arrancars in order to form an army and fight the Quincies.
And maybe I’m missing some details but that’s it. Sorry if this is too long and nonsensical, but I felt like sharing it. And if you have a different idea from mine or want to add something else, feel free to do so. I’m open for discussion.