One thing I'm more curious about, how many people have had their main character die? Only one of my characters died and it was due to old age in a playthrough I actually attempted world conquest. Even my companions died only a handful of times after the death rate was changed to 2ish% chance. I wish they would modify the death chance in battle (not auto-resolve) to 5%. I found 5% as the sweet spot between making death likely to occur a couple of times over a playthrough while not too often as it was with 10%.
I died three times as player in battles till now, in different campaigns. However I really don't like the "generation dynasty" aspects of BL, I don't marry and don't have children most of the times. I like the pressure that your companions (from whom some for me are kind of "friends/lovers") can die, so I always play with deaths enabled, which sadly starts the "getting older and get a family" mechanic. Detailed Character Creation saves me from the nonsense partly.
Before the modder sadly disappeared, I used "AI Values Life" and "Heroes Must Die" mods with higher death chances, so the AI (lords, companions) had a more dangerous life but lords also often avoided combat when having no chances. I did not start a new campaign after the death of my player character but reloaded, for me Ironman is not a challenge, but stupidity.
To OP: I don't think that the game is that easy. It is sometimes too easy and sometimes too harsh. It's surely boring and grindy after some point, and having big difficulty could make that even worse. To make games of a difficulty that very good experienced players see a big challenge is very difficult in itself. I'm not such a player and not a Warband vet (ok, I played it a bit, mostly 1257 AD) and I'm not very good in combat, especially without a shield, because I cannot parry well.
There is not much one can do to make the game more difficult (or interesting). Using Diplomacy and Separatism mods is self-explanatory, isn't it?
What else?:
1. I play on Bannerlord difficulty.
2. I use full RBM mod but without tournament module (so fully armored high tier enemies in tournaments from start, these bastards).
3. I upped T1 to T3 troop's skills and gave recruits shields, because AI often uses them.
4. T5 from normal and most T6 from noble troops are gone, to even the field and make the typical op player army more rare. Sometimes I read about problems with Filans or Crab Guards or so, don't know what that is.
5. I made me a custom faction (using Tetsojin mod, sorry) with a kind of Maya/Aztec/Toltec vibe, and I only use faction troops (10 foreign troops per party are allowed). My "Toltecs" have no to very few armor and no cavalry (with the exception of the top tier noble unit, medium armor and sitting on a giant armored war camel; party limit for this unit is 10).
It's (a bit) more difficult to win with such troops.
I don't think that "interesting new units" would change the field considerably. Only the addition of extremely op units (PoP style) or of fantasy elements, like magic, could have such impacts.
And even then it has to be a massive change. For example I gave soldiers with firepots (from OSA mod) to the Empire and the Aserai who can easily kill 10 to 20 troops with one throw. It's "funny" to suddenly see a big area of red fonts appearing, and it's just one enemy soldier. Sometimes they kill their own troops too, the morons, hehehe. There is no big impact to gameplay however (except the player gets a firepot and can use it in siege defenses ...).
What would change the game, as said by others before, were deeper tactical possibilities in battles. It needed a good AI first.