The preview art showing some X-Men planting seeds everywhere makes me think they won't be necessarily moving to a single place like Mars but creating "mutated" environments that mutants can live in (which follows into the "mutant clothes" thing Hickman proposed in interviews)
This sounds far more likely than 'X-Men Terraform Mars'.
Originally Posted by Wiccan
We used to all be mutants until Wanda said No More Mutants and turned our past history into comic books.
Originally Posted by The General, JLA #38
'Why?' Just to see the disappointment on your corn-fed, gee-whiz face, Superman. And because a great dark voice on the edge of nothing spoke to me and said you all had to die. There is no 'Why?'