I would like to see a full scale invasion from MANTIS. They're already an issue, so how about making them a threat.
Kill the MANTIS boss NPC and they stop spawning for half a day, only to return later in full force again.
MANTIS GOALS: Clear out all multi-block structures, if a multi-block is barricaded the MANTIS will tear it down to seize control. They do not destroy or repair buildings.
(Multi-block structures include Power Plant, Forts, Schools, Grocery Stores, Stadium and Mall.)