TL;DR list of proposed changes for the early-mid game monster distribution (justification below) -

Berserker monster line
Berserker - DL 4 > DL 6
Berserker Lord - DL 10, unchanged
Berserker Prince - DL 12 > DL 16
Berserker King - DL 16 > DL 24, Rarity 50 > Rarity 25
Berserker Emperor - DL 21 > DL 35

Werewolf monster line
Werewolf - DL 5, unchanged
Werewolf Lord - DL 12 > DL 18
Werewolf King - outside of scope of this RFE, but DL 30 > DL 35 appox

Mimic monster line
Mimic - DL 7, unchanged
Greater Mimic - DL 12, unchanged
Master Mimic - DL 16 > DL 20
Mimic Hivemind - DL 22 > DL 28

Ogre monster line
Ogre - DL 3, unchanged
Ogre Magus - DL 6 > DL 9
Ogre Lord - DL 15, unchanged
Ogre King - DL 20 > DL 24

Large Gnoll - Rarity 100 > Rarity 25
Goblin Berserker - Rarity 25 > Rarity 100

Flesh Golem - DL 7 > DL 8

Brown Bear - DL 9 > DL 6

Ice Lizard - DL 9 > DL 11

Fomorian Giant - DL 12 > DL 9

Stone Giant - DL 15 > DL 14
Shambling Mound - Rarity 100 > Rarity 25

Troll Chief - DL 17 > DL 14

Living Wall - Rarity 10 > Rarity 2

Frost Giant - DL 24 > DL 19
Fire Giant - DL 25 > DL 20

Chaos Servant - Rarity 10 > Rarity 100
Greater Chaos Servant - Rarity 25 > Rarity 100

---------------------------------------------------------------------

Goals -
1) Find out-of-level monsters, be it exceptionally weak DL 30 monsters or exceptionally strong DL 1 monsters
2) Find discrepancy in monster power and rarity of the monsters - assuming that more rare creatures in the same DL should be generally more dangerous
3) Suggest adjustments to the monsters

DL 4:
Out-of-level monsters - Berserker.
Out-of-rarity monsters - Large Gnoll.

Berserker have a huge damage and HP for level 4 monster, greatly outperforming any other monster - he is comparable to strong level 7 Outlaw Leader. Danger level of 6-7 would be much more appropriate.
Large Gnoll is 4 times more common after recent p22 change - while he is one of the strongest level 4 monsters already. Goblin Berserker instead of him would be good candidate on "common monster" of this DL.

DL 6:
Out-of-level monsters - Ogre Magus.

Ogre Magus have - best combat stats for level 6 monster, _and_ invisibility, _and_ regeneration, _and_ long-range targeted spell. Even though he is really rare, he will be in a better company with level 9 monsters, when PC's usually start to gather some items to generally resist such a special monsters.

DL 7:
Out-of-level monsters - Flesh Golem.

Even though he does not have PV, he hits quite hard and he is tough to kill - he is probally on par or even better then Clay Golem at level 9. This monster should be atleast level 8.

DL 9:
Out-of-level monsters - Ice Lizard, Brown Bear

Ice Lizards have slightly stronger fighter stats then usual monsters on this level and they have ice breath, which is hard to resist early. They are not really extraordinary strong, but strong enough to feel good at level 10-11.
Brown Bears are more rare, have weaker stats then Grizzly Bears, and yet they are still same level 9. They could serve a lot better as a rare strong level 6 monster instead of Ogre Magus.

DL 12:
Out-of-level monsters - Werewolf Lord, Berserker Prince, Fomorian Giant

Werewolf Lord dwarfs other level 12 monsters in combat power, and he is capable of filling a room with summoners. Even discarding that, sheer combat power is enough to put him on par with best level 16-18 monsters. He is most certanly does not belong to level 12 at all.
Berserker Prince have the same problem as a Berserker (and whole Berserker line as a whole) - they are strong. They are really strong for their level. Level 16 will suit him a lot better.
Fomorian Giant lack any special abilities and they lack combat power - they should be level 9 monsters, standing near Dark Orcs and Master Swordsmans.

DL 14,15:
Out-of-rarity monsters - Shambling Mound.

Shambling Mounds is another example of best monster in their DL, yet most common monster in their DL now. There is quite elegant solution - return Shambling Mound to original rarity, and lower Stone Giant level from 15 to 14 - he is not really that strong compared to others, and there are two common monsters on level 15.
Alternatively there are Chaos Servants/Greater Chaos Servants, which could fill "normal monster" part a lot better.

DL 16-20:
Out-of-level monsters - Berserker King (16), Master Mimic (16), Troll Chief (17), Ogre King (20)
Out-of-rarity monsters - Living Wall, Berserker King.

Berserker King - as usual, way too strong in combat stats for his level. Even level 24 will be okay for him, but lets be moderate and say that he belongs to level 21 atleast with old rarity, instead of berserker emperor. Insisting to preserve 50 rarity, as he have now - thats level 24 for sure.
Master Mimic - paralyzes, armor-pierces, blind rages and have enough HP to pull everything. Partially offsets by his rarity, but still level 20+ is more appropriate to him.
Troll Chief - weaker then a Troll King, but have same level as him. Troll King itself is a quite weak king compared to others - so atleast level reduction for Troll Chief to 14 would do.
Ogre King - tough, strong and not so rare monster, with stats of ~22 level one.

Living Wall - insanely strong for his level, same as a Wall Beast. However Wall Beast have rarity 2, which made it ok - but Living Wall have rarity 10, making it not-ok. It should be 2 too.

DL 21-27:
Out-of-level monsters - Berserker Emperor (21), Mimic Hivemind (22), Frost Giant (24), Fire Giant (25)

To introduce you how stupidly strong Berserker line of monsters are - lets say that Berserker Emperor (21) could probably put a fair fight to a Greater Fire Elemental (45). No further comments needed, this monster should be pushed atleast into 30-35 level range.
Mimic Hiveminds - same as Master Mimics, just worse. Level 26 or Level 28 would put him in a great company.
Frost Giant - he is a lot weaker then Frost Giant Berserker, but have a same level 24, despite being A LOT weaker.
Fire Giant - almost the same as Frost Giant, just a little bit stronger. This pair of giants should be reduced to level 18/19 or 19/20, and switched with Ogre King instead.

DL 28-32, aka "The Wall" problem.

Let me introduce "The Wall" problem, which is probably intended and a good design decision in 1.1.1 times, but not with new quest being added and monster distribution being changed.

The Wall - A HUGE amount of end-game monsters dropped on a player at once, right after the wall of flames at DL 27, which allows PC access to DL 28+ dungeons.

Basically everything after level 28 is end-game, with occasional single strong monster added every 5 levels. Ancient dragons, unicorns, werewolf kings and ogre emperors, master liches. Majority of the monster lines ends at the 28-32 level. There is a need for major rework here to maintain flow of the game - this will be discuissed later in a separate RFE among with DL 28+ monsters.