I think there's enough to Aloy to warrant going back to her: she's discovered who's her "mother", but she probably would want to actually know her mother. In a sense, it's another kind of quest for identity. Knowing who Lis was entails understanding her world, piecing together the lost story. A quest for APOLLO.
Aloy, like Lis, seems to be not only absurdly intelligent, but curious, and curiosity can be an immense drive. She probably would also see rebuilding GAIA as her personal responsibility.
Odyssey will almost certainly play a big part in the sequel. Maybe it failed, but not the way we think - maybe it stayed in stasis for what would be the duration of the trip, then it started doing its thing. New humans wake up, receive all the unfiltered knowledge about Earth (that probably would lead them to believe some really weird shit about what went down around here), find out they're still around the SS and go "we should go back to Earth, but lets, dunno, reset GAIA and get her to rebuild it in a way to avoid the shit that happened before".
They arrive and well, things aren't exactly like they thought they'd be. Not at all.