So simply factor that in. Increase the gravity global (if gravity's a global, it's been literally years so I can't remember). Failing that, just halve the jump height and make vehicles weigh more to simulate the effect of increased gravitational pull in one direction. Projectiles and grenades can arc twice as much. There you have it - an original sky, and an original factor to change the combat up a little. Rather than automatically assuming it'll break the map, think more creatively. Low-grav games seem to provide a fun change in pacing, why not go the other way for a similar (yet opposite) effect on play?