I think what's most important in any tree is a variety of skills and different mechanics. That's what makes it fun. And that means 1-2 AoE skills, 1-2 buffs/Passives, 1-2 single target skills.
And following that principle, the game already is working great. You usually start the tree off with a single target skill. Early early game density allows to rely on single target only until ~ lvl 6 where you get a strong passive/buff or a small AoE or a 1hit single target skill. Then from lvl 12 on another greater AoE should get into the tree. Pls keep that in mind for future theory crafting

Which skill I personally dislike the most is the poison skill Angel of Death on Necro. Because you have to depend on minions to tank for you, so the enemies stay in one spot and you can jump in and 1hit anything, then move away from the enemies and jump in again. The jump mechanic is just really really bad for a single target melee strike that doesn't have 100% chance to hit. Because it's a) time wasting as the killspeed is really slow upon reaching cold plains where you get swarmed b) very boring to play c) imo a misconception in the first place cause the enemies move way faster than you when you jump so heavily rely on someone/something tanking for you d) the jump just looks aweful and e) why the hell can some Necromancer who relies on his minions and thus shouldn't be physically fit, suddenly jump 5 meters into the air?
I mean I can see why the jump was chosen. You're invulnerable for the time you're in the air, and thus can simple 1shot most things early game which fits the mechanic of supporting his minions. But jumping in general is just bad in D2 for the reasons I listed.
If this jump mechanic should stay, then make it at least a very small AoE near your target location without the attack animation when you drop to the ground. Else I'd love to see the necro charge (like the pala in vanilla d2) for the poison strike.
The other thing I'd change is the AS of the wolf form of the druid in early game. I couldn't get myself to play this one any further because the as of the wolf is so slow... You attack even faster when you're not buffed... And you try to do the lc0 and think to yourself: 'why exactly did I skill this wolf form? Am I actually paying skillpoints to get a disadvantage in attack speed? Why should I even use this morph any longer? why shouldn't I just start playing some other char or go for treewarden?'
Well, yeah I stopped playing that one after lvl 5 cause I just didn't see the fun in attacking once every 2 seconds just to have no mana at all when I can just use treewarden and attack like 5 times a second. Like I said, I dunno if it becomes any better, and I didn't want to find out, but early early game it sucks and takes away the fun to explore this skilltree. Just making the wolf form fast enough to keep up with the as of the unbuffed druid is completely sufficient.
That's about it. I wouldn't want to change anything else. The game is well balanced and doesn't need any greater changes in the skilltrees.
What I really like to see though is more oskills cause I love crafting a nice weapon with an oskill and make builds around these oskills.