The ending of X8 seemed to suggest that, if it came to the day when there would be no use for reploids, Zero would not go quietly. I think there's a lot of potential to make a really deep, character-driven story here, where perhaps the world's government or whatever's left on Earth decides to take in all old-era reploids and decommission them, and X actually fights for the side of the humans. X is tired of fighting, tired of war, and just wants his "duty" to be over, so he's accepted his fate and is willing to be decommissioned himself, but Zero starts a rebellion against the humans (with a group of reploids also deemed "Mavericks") and X is sent to take them down. After all the build-up of how X is talking about how he'll have to kill Zero one day, I'd love for it to be because of a philosophical disagreement, and not because Zero gets "infected" or some stupid [parasitic bomb] like that. There's actually a lot of thematic depth here about free will, right to life, a child's responsibility to his parents', etc. all of which make for a great classic science fiction story. I think it'd be a great way for the series to go out, personally, though I'm not sure which ending I'd like most: one where X dies, or one where he lives.