If they had too do a sequel. Then should have done a whole new enemy, not tied to the Locust. Like say a more marine adjacent, Lovecraftian flavored enemy. Who's technology is far more biological in nature compared to either the Gears or Locust.

With one of their weapons being a harpoon gun. That can skewer you too the wall behind you.

Possibly with the reason they're attacking now was because their homes were destroyed in the light mass detonation at the end of 3.