Hi,
recently discovered this and I have to say well done. I played every single mod there is for GD and this one is very clean, not a single bug i encountered, full of good ideas on D2 skills implementation. Thumbs up to you!
Now I see you re looking for some ideas and after spending some time with the classes I actually got plenty 
See there is major difference in design of character progression between D2 and GD, making your work quite difficult.
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in D2 each new tier skill is vastly superior to the skill from previous tier (fireball is better than fire bolt, phoenix strike is better then blades of ice …). Together with the fact that you have only two active skill slots in D2 and only some 100 skill points, the game design assumes you will pick 3 - 4 top tier skills and thats your build. Later on they added synergies.
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In GD you have 11 active slots, 250 skill points, and skill modifiers. With each tier you unlock you open up a new skill, bot also a mod for a skill lower down the tree which makes that one more powerful, ultimately making all the skills across all the classes viable for the entire game. Also, as with D2 synergies, game rewards you if you invest in entire skill trees
To make D2 skills a lot more “grim dawned”, i would consider doing things like these:
Assassin -
Entire skill line Fist of Fire - Phoenix strike. Either make Fist of Fire active skill and everything else modified (Claws of Thunder adds Lighting dmg to Fist of Fire, Phoenix strike adds + all elemental damage to fists of fire. All those nice novas you have there could be cumulative), or make this entire tree as a passive Weapon pools skills (Assassin doesnt have any but I think should have). In this way you will reward players if they invest in entire skill tree. + all skills items will have MUCH biggers impact on character strength.
Tiger strike + Cobra strike - Make CS modifier for TS, that will make TS powerful life sucking ability useful for entire game.
Dragon talon skill tree - Make this like Nightblade shadow strike line - Dragon tail, single target kick. Dragon talon mod adding AoE, +fire damage. Dragon flight mod adding teleport and + % fire damage + increasing AoE
Venom - entire assassin is done around elemental damage, both weapons skills and traps. Make this one passive “Elemental Blades” adding + elemental dmg and +% elemental damage. It will empower entire class
Other skill trees in Assa are good
Sorceress
This is all done well! You have made good decisions here like making fireball mod for firebolt. Same with Ice bolt tree, Frost Armor skill tree … nothing to add, lots of useful things to pick
Paladin
This is an extreme example D2 vs GD design differences. 80% of the skills are exclusive and each tier is vastly more powerful than the previous one. What can be done to make it more diverse?
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Remove salvation, Keep Resist fire and make Resist shock and Resist frost mods for RF. Thus by selecting all 3 skills you will get same effect as picking salvation. You will reward player for investing in entire tree. Or you can lso keep Salvation as mod for Resist fire, adding + Chaos and Aether resists to the mix. That would be actually useful since there is crapload of elemental resists everywhere anyway
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Holy fire, frost, shock tree - same as above, keep only holy fire as an active skill and make holy shock and frost as mods, adding their damage types to holy fire.
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Might - Blessed Aim …. Fanaticism - Keep only might, blessed aim mod adding attack speed and +crit damage, fanaticism +all damage +total speed
Doing this will drastically decrease number of exclusives and make each one of them vastly more useful
Barbarian
Generally good, same as sorceress, lots of passives, lots of things to pick
Druid
Good, only thing i would change is - possibly add in there somewhere a skill to strengthen to spell casting abilities, like bear improves the melee.
Amazon
Havent played yet but just looking at it seems fine
Hope i managed to explain what i mean by those design differences between D2 and GD which makes it impractical to clone the skill trees 1:1 and laos hope this will give you some further ideas on how to progress with you mod.
Edit: - As a golden rule, when you see that one active skill cancels out another active skill in GD, it is a good indication it should become a mod rather then a skill. Eg. in Paladin tree, Salvation cancels out Resist fire (it is so vastly superior in a sense that it has the exact same function + a lot more) indicates Salvation should be a mod rather then individual skill. Exactly the same with Phoenix strike and all the previous skills in the Fists of Fire tree. I think if you take a look at all those classes with the eyes configured like this, you´ll get a millions ideas.
Happy moding 
