I got a few ideas on how to make some aspects harder :)

1. Silent Mobs
   Mobs that do not emit any sound, be it growling or steps. Or a chance that a sound is being deafened.
2. "Not sculking"
   While a Warden is alive, each Shrieker will either be rebuilt or prevented from breaking
3. Crawling Mobs
   Mobs can now enter a crawling like state where they can potentially fit through 1 block gaps.
   Also occasionally enter this state while fighting a player to try and avoid an attack, mostly when the player jumps
4. Ninja Mobs
   Mobs that will throw a Splash Potion of Invis below them, turning themselves and nearby Mobs invisible. Particles disabled.
5. "Lead me not"
   When the mob successfully hits the player they put on a leash on the player preventing them to run away further than 6 block direct distance and pull the player back when somehow succeeding. Breaks when further than 20 block distance

Silent Mobs

Can already be done extensively with Mobs Properties Randomness, so no. Will add the chance to not emit a sound to MPR (not sure how feasible it is)

"Not sculking"

Don't understand this one. The warden has already been buffed.

Crawling Mobs

I had already thought about this, never started working on it but I'm afraid is not really possible with how mobs are coded.
Will definitely explore the idea of stafing to avoid attacks (like skellys already do)

Ninja Mobs

Not a job for mobs, more like a job for Witches.
Also particles disabled is 100% unfair.
Also also you can make a mob have a chance to spawn with potion effects with MPR.

"Lead me not"

Quite sure also this is not possible, but will definitely explore and try to implement.

If you do manage to get that last one working, you should be able to use leads on players LOL

"Not sculking"

Don't understand this one. The warden has already been buffed.

The idea behind it is that once a player has summoned a Warden they cannot brute-force deny the spawning of other Wardens.

Crawling Mobs

I had already thought about this, never started working on it but I'm afraid is not really possible with how mobs are coded.

Yeah the crawling itself would be hard to impossible, I was also thinking for alternatives.
One I had in mind was a "Morph" like ability where they would half their size

Ninja Mobs

Not a job for mobs, more like a job for Witches.
Also particles disabled is 100% unfair.

Might be something for Witches, true.
The Particles disabled admittedly a bit unfair, but steps should still be played, so their sounds like growling.
Truly unfair it gets when combined with Silent Mobs :' )