I think even tuning up the enemy ai would do wonders for challenge. Take a game like the Division 2 for example. I don't get bored of the combat because the enemy ai is actually pretty good and will strive to use their kits to do as much damage as possible to the player.
I have to agree. The AI in the Division 2 is amazing. So much better than Destiny's "lets run in a circle around a rock" enemies.