TL;DR
A new class of biters for later game. Chameleon Biters that can infiltrate your base. Only visible whilst they are moving. When still they look like a rock, tree, inserter, assembler etc.What ?
I suggest a new late game development of the biters. Stealth biters. They should be able to sneak into bases and cause damage, intelligent damage, like cutting power lines, blowing up train tracks, or even buidlings. Damage should escalate the longer they are "on the loose".But in order to create extra stress on the engineer, they should be inviisble, when not moving and when not within line of sight of the engineer. So the player would have to keep watch out for sudden ghost red dots on the radar, for things out of place.
This could lead to new uses for combat robots, who might be retrained as sentry robots, to try to spot the infiltrators.
I believe that tis could add a new level of threat for those who want to fight an enemy, and for those that just want to build, well we have biters turned off anyway, don't we?