Weighted monster distribution was already RFE's and discuissed on this forums - http://www.adom.de/forums/project.php?issueid=1116

In the same RFE I've experimented with executable modifications to find out optimal adjustments.

Majority of the players agree, that early game (pre-animated forest/dwarven halls, DL - 1-~14) do not need any adjustments - they are fine as it is, and the first locations to experience slight increase in high-level monsters would be ToHK and CoC level below Animated Forest. Late-game is probably best defined as diving past Casino to Air/Earth/Mana Temples, guaranteed Greater Vault and perhaps Blue Dragon Caves.

Unseen complications that i found while experimenting -
1) Level 1/2/3/4/5 mosnters have 14%/14%/9%/6%/12% spawn chance respectively. First 5 levels total have 55% spawn chance. If they are just removed from spawn - it feels like a different game, and i'm not sure if it is better.
2) Some high danger monsters have extremely low spawn chance - for example, great red wyrms have 2 spawn chance. Even emperor liches which could be as dangerous if not more - have 10 spawn chance. No amount of general spawn chance changes depending on DL would fix this.
3) Some monsters rarity does not change depending on their hierarchy - for example baby dragons, dragons and ancient dragons all have spwan chance of 10. If is probably too low for baby dragons and for normal dragons, and maybe a little bit high for ancient dragons.

In-depth analysis of 2) and 3) would be in separate RFE if needed.

Issues with proposed #2601 RFE -
4) _very_ steep change in monster distribution due to 1) -
DL 10 with lvl 10 PC would be same as before.
DL 15 with lvl 15 PC would feel quite different already due to halving rarity of low-lvl monsters and a lot of higher-level monsters appearing.
DL 20 with lvl 20 PC.... would feel almost same as before, as majority of possible monsters now have their spawn rate halved! Only change would be occasional high-level monsters appearing more frequently.
DL 25 with lvl 25 PC - another steep step, which would be _very_ drastic and _very_ sudden - one level you are fighting with goblins and just several levels below you cant find nothing familiar. It does not help that DL20-30 is _one_ ride to the depth of CoC usually.
DL 30 with lvl 30 PC - i think that would be already almost impossible to play and another very sudden change in monster distribution. Also it would make passage through Dwarven Halls impossible for most PCs.
DL 36 with lvl 36 PC (where endgame starts, before Air Temple) - unplayable nightmare. let me elaborate - you almost cant spawn low-level monsters, monsters with DL 16-26 are half as likely to be encountered, DL 26-36 is full chance and DL 37+ cannot spawn.
Just fun fact - most common monsters at this level would be - Fire Giant, Frost Giant, Titan. Yes, Titan.

My suggestion for monster distribution - reduce spawn rate of monsters 15 lvl or more below current danger level (on lvl 16 PC and lvl 16 DL - goblins will start to be less common) - by 6% per one DL, stacking multiplicatively.
This way 500 rarity goblin on DL 15 will be ~270 rarity on DL 25, ~200 rarity on DL 30, ~105 rarity on DL 40.
This will fix too sudden changes on monster rarity (which will be especially drastic on DL 20-25 and 10-15) and still leave late game playable, which i doubt it will be with current proposal.

Additional tweaking of high-level monsters rarity still will be needed, however.