spoiler alert
An enemy that would just require a new turret or just have more health would also get boring and old fast. (flying enemies -> Anti air, Water based -> submerged turrets).What I think would be interesting, would a native that would take a new way, and more strategic way, to eliminate their bases.
What I'm thinking would be an enemy that would have near human intelligence, they would be able take structures from your base, mining drills, assembling machines (limited to the item they were creating before they took the assembling machine) and other structures that would be necessary to an automated base (or they could make them by themselves), and they would be able to take those structures back to their spawners. They would then place down the structures with basic and rudimentary design to attempt to create machines that can fight back against you and your ever expanding base (or something like that).
Basically I want a mod for a specific type of creature that are able to create their on factories and defenses to protects themselves from you.