Warlocks have some serious issues in classic, the main being there only really being one spec for pve (Ruin) and the other trees simply existing to support those. This is also a glass cannon spec rendering you useless in pvp, there is practically no viability for destro in pvp, beyond seduce soul fire meme stuff.
Demonology has no viability in terms of actual gameplay, it exists merely to sac your succubus/Imp for pve, or soul link/feldom for pvp. Warlocks have no cleave, their aoe is far inferior to mage. For comparison arcane explosion has both crit scaling and better spell power scaling compared to hellfire, which leave you standing still, damages you, cannot crit and you have no means to escape like with blink, nova, slows or ice block.
My suggested changes would be to make each spec viable and have a distinct role, I will not cover demonology fully as it recovers a major rework of that should also affect BM/Hunter pets, mainly making them scale with both the masters stats and buffs.
First what needs to be done is making feldom and master summoner baseline and changing the talents to improved feldom with a 5 min cd at 3/3. Demonic embrace should not lower spirit. Demonic sacrifice should also be baseline at half the effectiveness, with improved demonic sacrifice deeper in demonology. The improvement talents for imp, succubus and voidwalker should be merged and also have bonuses to felhunter. Improved spellstone should be replaced by either Felguard or making Infernal a summonable pet instead (without enslavement and having to be outdoors). Spellstones and healthstones should be given a playstyle effect for demonology, i.e affecting mana, damage and the pet.
There is simply too much that would have to be reworked in demonology for me to offer further suggestions.
However the following baseline changes should be applied to warlocks:
- Feldom baseline & a halved form of demonic sacrifice baseline.
- Demon Armor should give 33% of spirit as spellpower.
- Hellfire coefficient should be raised from 33% (over 15s) to 100% (This is still lower than arcane explosion).
- Rain of Fire’s coefficient should be raised to 100% (Currently it does less damage than blizzard does, which also has the best slow in the game)
- Drain Soul’s duration should be lowered to 5 seconds and deal 50% more damage to targets at or below 20% hp
- Drain Mana should scale with spell power (50%)
- Curse of Shadows & Curse of Elements should be merged into Curse of Chaos and affect all magic damage instead.
- Curse of Chaos & Curse of Recklessness should reduce the cast time of your next soulfire or shadowbolt by 1.5s when cast against a target above your level.
- Drain Soul damage ticks should have a chance for soul shard (0% when you have no none soulbag slots)
Now for the real talent tree changes.
Affliction:
Improved curse of agony should be raised (from 2%, 4% and 6%) to 5%, 10% and 15% per point.
Nightfall proc chance should also have a chance to proc from drain soul and the effect should be raised to 3% per talent point (from 2%)
Dark Pact should be replaced by Harvest:
Affliction spells are now able to critically hit. Drain Soul affects all targets affected by your damage over time effects and Drain Soul deals 33% more damage per damage over time effect on your target and costs 10% more mana per active damage over time effect.
Destruction:
Bane and Cataclysm should switch places. Bane’s soul fire cast reduction should be increased from 0.4s per point to 0.5s per point. Cataclysm should be raised to 2% per point and also reduce threat by 3% per point.
Aftermath should be changed to be 100% but only affect shadowbolt, conflagrate and soulfire, apply a slow at 10% per point for 6s.
Intensity should affect all destruction spells and it should include the effects of Destructive Reach.
Improved Immolate should take Destructive’s reach spot and not unlock any new talents.
Pyroclasm should be a slow instead (25% per point).
Ruin and Conflagrate should switch places.
Conflagrate should have it’s spell coefficient raised to 60% (from 43%) and if it critically hits it should affect the target with a conflagrate dot, dealing immolate tick damage to all nearby enemies (5 yd) every 2s for 6s.
The talent leading to the now new last talent (Ruin) should be replaced by Improved Soul Fire:
Soul fire no longer requires a soul shard and has no cooldown.
Ruin should be changed to also allow hellfire, rain of fire and immolate to crit.
Edit: Some further points about demonlogy.
Summoning pets & sacrificing them should be part of an active playstyle (Ideally to buff yourself temporarily and your infernal. Sacrificing an imp could make your infernal do significant aoe damage, sacrificign a succubus would make it deal single target shadow damage. Right now and until like Legion? you basically just summon a pet once and either sac it or keep it around, basically just a cooler version of a buff, not an active part of the playstyle that could make warlocks unique.