I’m a huge fan of the idea of giving enemies perks similar to player mercs. The Long War mod for XCOM:2014 did the that and ended up providing both increased difficulty and diversity. Enemies were tougher, fought differently, and had to be approached on a case by case basis, all of which is excellent in my eyes.
Having a system in Battle Brothers where perked up enemies began cycling in later in the game could do a lot for the difficulty curve lategame.
