"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.
Rampant makes the enemies smarter, but not necessarily harder. If you've learned how to defend from the biters in vanilla, some of the new tactics will catch you by surprise. But the answer is still the same: build up your defenses to the point where no attack wave stands a chance.
Rampant has an option for exotic biter types, now turned on by default. It's even recommended to play it with Rampant Arsenal, which adds a bunch of extra weapons.
Rampant + deathworld (I think is the combo) is the way to go if you want a real challenge, because you need to build defenses that do not use too many resources.
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.