I am not sure what exactly u ask here, maybe its due to my english being bad but ill answer you the way i understand it
What im going to say isnt item related only but generally related to Diablo 2-like games.
With all honesty i think the era of mindless hack & slash is comming to an end and this is the most important aspect to why ppl get disappointed from mods even if they dont admit it. H&S games tend to have no meaning and that itsself is boring. Realism and following the core principles of what makes your game what it is are crucial for any good game. If your games lacks principles that determine its world, classes, skills, etc etc then its a bad game.
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Monsters -> Quality over QUANTITY
Dont try to fill the map with monsters, instead try to make monsters different depending on their type and background story. Give them different stats and abilities BUT dont give them random stats and abilities for the sake of differentiation... instead give them stats/abilities that fit WHAT THEY ARE.
For example even at the very starting area in normal you can have one monster with 10-15 dmg and another with 40-50 dmg (the numbers are pure examples to show you the difference), or one monster types with very high speed (maybe they are assassin type or whatever ) and monsters with very slow speed (zombies etc), just dont be afraid to give them their own characteristics, cause will make your game interesting and non-linear.
As i said, dont fill every area with tonz of monsters, maybe some areas have only a specific type of monster and they arent that dense?
OFc all this kind of changes require vision, creativity and something like that is often way harder to find than just increase all monsters dmg by 50% cause they are now in Nightmare and not Normal...
Also, Ai improvements are always more than welcomed, especially when combined with low density monster population.
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Items -> Property quality over quantity.
When ppl think of D2 they think of items with 23021030 properties on them and this is exactly what kills item-isation because items start to feel the same with slight difference and they totally use their charm, you no longer hyped to see your next rare or unique cause u know most of the stats will be the same stats your previous rare, unique had. So itemisation becomes boring hence the game thats all about items loses its meaning, hence rip D2 mods
What you should do for example is:
Rare: 1 to 3 properties.
Considering there are so many properties in the game, you will always look forward to pick up your next rare cause it might have those 3 stats that you so much look for or maybe one stat x3. Example, you might get 5% Deadly Strike, 1% Life Steal and 1% Mana Steal or get 15% Deadly Strike (again numbers and properties mentioned are pure examples).
Unique
The word UNIQUE itsself should say it all.... You can have a unique item with 10 properties or just 1 property or maybe even uniques with negative properties?
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Skills
Make sure the skills you add to the class actually fit the class.
I hope i helped in some way.