Right now, each mission has a fixed map size. For X-Com veterans, this tends to make missions a bit more predictable.
Therefore perhaps we could have the map size somewhat randomized, just like the number of aliens is randomized? This would require some changes to the ruleset, for example:
alienDeployments:
- type: STR_MEDIUM_SCOUT
minWidth: 40
maxWidth: 50
minLength: 40
maxLength: 50
height: 4
If the old format is used (just width and length), the map still has fixed dimensions.
It is also possible to correlate bigger maps with more aliens, but I think it's too tricky to balance, and just unnecessary.