Amazon:
Poison Javelin: Needs Attack Rating like the other attack skills.
Cold/Fire/Magic Arrow: Make them more competative with other skill options. (EG: Give Multishot synergies from these skills adding Cold/Fire/Magic damage respectively. Or make it so each point in the skill gives +3% Change Physical to Elemental Damage so that level 20 is 60% conversion to elemental damage and level 40 120% conversion to elemental damage)
Sorceress:
Frost Nova: Reduce synergies to only Shiver Armor but same total max increase to damage from synergies as current (makes it more defensive oriented option compared to Nova)
Inferno: Add Monster Flees while taking damage, giving Fire Sorceress a method of crowd control. Add damage over time per plume that strikes (ie make it act like a napalm flamethrower), this will allow for more consistent damage.
Change Blaze synergy from Warmth to Enchant Weapon, change Enchant Weapon synergy to Blaze. Warmth is a wasted synergy for Blaze/Enchant Weapon because you are not constantly casting. FRW from Blaze works much better with supporting Enchant Weapon and vice versa. Instead make Warmth a synergy to Frozen/Shiver/Chilling Armor, granting the armors +1% Faster Cast while active per point in Warmth, thus a max of +20% Faster Cast.
Necromancer:
Fire Golem: Synergize with Lower Resist and Skeleton Mages instead of other Golems
Blood Golem: Synergize with Bone spells instead of other Golems
Poison Dagger: Make this viable. Maybe synergize with Dim Vision somehow?
Curse tree in general has too many one point wonders. Difficult to balance correctly, but maybe fewer synergies and changes to the Curses so that it’s worth putting points into them.
Paladin:
Sanctuary Aura: Apply to Demons as well as Undead. Trigger pulse (knockback and damage) on striking (this will make it synergize well with Charge paladins).
Conversion: Give synergies with Thorns
Blessed Aim: Add % chance to pierce target. Maybe as a synergy from Concentration?
Barbarian:
Increased Stamina: Change to Increased Resilience, Add +2% Faster Hit Recovery per level.
Stun: Replace with something else, currently the most useless skill in game. Maybe a ranged weapon ability to be used with either a bow or a single throwing weapon? Or maybe a counterstrike ability based on responding to attacks after blocking and/or hit recovery?
Concentrate: Give it some kind of spillover/cleave ability? Maybe if a Concentrate attack is the killing blow, you trigger a bonus attack on an enemy in range?
Druid:
Arctic Blast: Give Freeze Effect instead of Chill effect. Each ice shard that strikes the enemy should reduce enemy resistance by -1%, thus as more and more shards strike the enemy Cold Resistance will drop.
Hunger: Replace with Ice Claws (like Fire Claws but Cold damage instead).
Vines: Give each vine an enemy debuffing effect. Give Poison Vine a Slow Enemy aura. Give Carrion Vine a -% Enemy AR and -% Enemy Damage effect. Give Solar Vine a -% Enemy All (Fire/Cold/Lightning/Poison) Resistance.
Assassin:
Psychic Hammer: Triple the damage increase (for both Magic and Physical) per a point in Psychic Hammer. That would result in Level 56 Psychic Hammer doing approximately 520-640 damage (half physical and half magic). Compare this to level 54 Holy Bolt that does 838-956 magic damage (also piercing but limited to Undead and Demons). Then also give Psychic Hammer a synergy of +50% damage per a point in Mind Blast (comparable to the +50% damage increase Holy Bolt gets from Fist of the Heavens).
Mind Blast: Give Mind Blast a synergy of +10% damage per a point in Psychic Hammer. That should result in a level 50 Mind Blast doing 1008-1038 Physical Damage with max synergies.
Fix Next Hit Delay: Give each skill/effect a separate Next Hit Delay instead of sharing it across skills (similar to how casting delay used to be across all spell and was changed to applying separately to each spell). Next Hit delay hurts many builds, but none more than the Assassin. Also remove NHD from Blade Sentinel, or at least reduce it to 4 frames.