"More interesting aliens and strategies for dealing with them. After awhile, the aliens start to feel like a repetitive grind. They always behave predictably and the solution to them is pretty much always the same."
Try the Rampant mod, if you want a challenge. There are a bunch of other mods that add different/tougher enemies too.
That said, the focus of Factorio was never combat, it's building. If you want better combat there are many other games that focus on that.