I played game on self-made Tactician+ difficulty and it was more interesting in such a way: - No consumables in combat allowed (potions / scrolls / arrows / bombs) etc. - No buffing elixirs and weapon coatings allowed. - Potential sparing of enemies that I did not think deserved to die - for example, I tricked Hag's Masks in Act 1 to pass through them (which led to necessity to fight them alongside the Hag) and knocked all of them out during her battle instead of killing them (since they were just mind controlled pawns) or for example managed to knock out all of Flaming Fists in Wyrm's Rock Fortress instead of killing them (since they just tried to protect their "Archduke"). While that does not sound difficulty increasing, it actually is - you need to manage attacks and especially offensive spells much more effectively to not accidentally kill targeted enemies by them, especially since many of the spells can RNG crit.
As such, if you want to have increased difficulty in pure unmoded game, you can play with such and other restrictions.
As for the topic - I agree, adding 4th difficulty could be nice.
