(For the reference, I'm a Hunter main who plays Warlock as a hobby.)
Here's a comparison of some of the Warlock abilities between D1 and D2.
So, in D1, Voidwalker specifically, you had Energy Drain and Life Steal/Surge to quickly recover and get back in the action with grenades. And getting grenade kills would, in return, give you Energy Drain again (given that you Embraced the Void). Now, Voidwalkers with the Attunement of Chaos get a worse version of Energy Drain (with the melee), and get few, but damaging grenades. I believe this should be reworked to where instead of draining your super to overcharge your grenade, you should be able to drain your super to "recharge" your grenade. Giving back the synergy that Voidwalker was known for.
Now moving on to the solar subclasses. In D1, the most well-known and mostly perceived to be the main melee ability for Sunsinger was Flame Shield. (This granted an overshield after meleeing an enemy.) But when we transferred to D2, we got Dawnblade, and the melee ability for the Attunement of Flame was a rework of Brimstone... (and the melee ability for Attunement of Sky was a rework of Voidwalker's Surge, but I'm gonna focus on Brimstone here cause Surge is better than Brimstone.) So, brimstone... the least popular Sunsinger melee ability in D1, even though it had a reduced cooldown opposed to the base 60 seconds, now has a 60-second cooldown, only so that you can cause a Firefly/Dragonfly-like perk IF you get a kill. Plus you can't even generate orbs anymore on kills, so... RIP. So, just, take it that I want to see Flame Shield back.
I know I left out Stormcaller, but that's just because I have little experience with the subject.
Now, here's just an idea, don't judge the rest of my post according to this: [spoiler]If Titans have Ward of Dawn, let the Attunement of Sky on Dawnblade bring back self-res, and make it so that when you activate your super from beyond the grave, you would be in Radiance instead of Daybreak. That way there is a reason to activate it while you're alive, instead of always waiting until you're dead to activate it. ("It" being the super.)[/spoiler]
Edit: Changed title, it was misleading, and also restructured my thoughts within this post.
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Sunsinger was garbage and fundementally broken in D1. Self rez broke the game rules in pve and should never come back. If dying was how your class was useful that's a problem. end of. DoT was over tuned and then it became useless via nerfs. leaving you with a semi decent melee that basically won melee engagements for you. Dawnblade is MUCH much better. You finally have real killing power in pve with it. One is more about killing and the other is more about mobility. If D2 warlocks had the self sustainability of D1 they would be broken in pvp. I vastly prefer D2's voidwalker because it ups my AOE damage/kill potential with one tree. and I still have a more "survival" loadout with the other but it's actually balanced. Not to mention they got to keep blink. and the very interesting charge nade mechanic. Warlocks are the mobile damage class now. And as a hunter main (who played a boat load of all 3 classes in the last game and this game) it actually annoys me.