Some way for ice to replenish on the map might be nice. Even in the normal game I noticed if you've got a couple of greenhouse modules on the go, you use the stuff up pretty quick, and are forced to go further and further to find more. I can see in Endless mode how you'd eventually just run out, and as a result run out of food as well.
Alternatively, some sort of collecter the player could craft to extract moisture from the air and turn it into ice would solve the issue too. Nothing too easy to build, since you want the player to be hunting for the stuff in the early game.