There are a few mods out there that aim to make the game harder in various ways by changing all kinds of stuff, but if you’re just looking to make combat more fun and immersive, have a look at these three that I present in this video; first I show the unmodded state, then with combat tweaks (among other things, lowering of master strike probability) and removal of slowmotion, and lastly with the two aforementioned plus a mod that removes most of the HUD:
These mods could be paired with some tweaks to player/AI health and armour functionality to make the game harder (and more realistic/consistent), especially in late game. I have done this in my personal collection of mods.
If there’s any interest in it, I might be persuaded to release a clean but comprehensive realism/immersion/hardcore mod at some point (perhaps after the next big patch).