Sorry for the incoming wall of text but I have lot of experience with game design and balancing so I want to get this out of my chest.
Pandemic - Shadowflame should be able to crit too.
Lake of Fire - should be either straight up buff to Rain of Fire and Hellfire, or should come in multiple stacks each applying with tick of damage, once max stacks is achieved one tick of damage should be enough to refresh the duration, that way there is still the initial cost but it becomes much more viable in longer fight.
Master Channeler - should apply to health funnel too, this ability is just clunky, very slow to heal the pet, you need to channel it and in the SOD quest for the rune we actually had to use health funnel not drain life.
Soul Siphon - as mentioned before by others it is just very weak and absolutely pathetic in comparison to master channeler which is in the same slot! Execute mechanic (or anti execute, lower your health higher the heal) of some sort could work or general buff to the damage/heal + ability to crit (+20% damage per stack, stacks up to 100% - this would put it close to other fillers) that could turn drain life into affli filler.
Demonic Tactic - this is just not strong enough and it gets relatively less effective more crit you have from gear and buffs, in general it might result in +3-9% total damage buff depending on talents and other runes, compare that to master channeler that can be 15-20% of your DPS in 1v1 situation and can proc Synergy rune and Nightfall talent. It should be either combined with Demonic Pact, or buffed to +15-20% crit or + 10%crit +10% haste or +100% scaling of the pet with Warlocks crit chance.
Invocation - should trigger the effect every time the dot is refreshed not just when it is finishing - it would result in comparable damage to the searing pain from Meta, this would give lock viable alternative to Meta, because lets face it SOD is clearly about instant spells, movement and burst, there is not much time for hard cast gameplay, especially in PVP, game is just too bursty.
Everlasting Affliction - on top of its current effect it should cause Shadow Bolt, Shadow Cleave and Shadow Bolt Volley (and maybe Haunt) to apply corruption on the target. It would of course synergy with invocation and as a result could lead to some interesting builds (experimentation in SoD) for Affli and Demo.
Demonic Grace - remove the GCD please, maybe make it usable in Meta even without pet, possibly some scaling with some of the Demo talent(s) - for example each point in improved succubus would increase duration by 1 second, or something.
Demonic Pact - needs a buff of some sort, perhaps increase to spell damage done for the lock like in WOTLK on top of the current effect, maybe percentage of your spell power (30-100%) applied as AP to your pet when it procs, or again scaling with some of the Demo talent(s) - passive increase in effect when you unlock demonic sacrifice or master demonologist, etc.
Demonic Knowledge - is fine, maybe could scale with some late demo talent.
Dance of the Wicked - the mana regen needs buff or scaling with talents, maybe weaker version of the buff should not overwrite stronger buff unless its like 3 seconds till the end of the buff.
Shadowflame - is pretty good at the moment, however it could benefit from pandemic talent and maybe backdraft could be triggerable by this spell too not just by Conflagrate (so you could go for weaker version of the buff with 10 sec duration and 15 sec cooldown if you dont go full destro).
Unstable Affliction - DOT damage should be buffed and / or it should be at least instant or put a twist on it make the silence and nuke damage proc when the DOT runs out if it does not get dispelled in the mean time.
Felguard - it should deal more base damage than Succubus at least but if devs want to avoid it becoming default pet for all specs it could just scale much better with Demo talents, for example Unholy Power could have double effect on it, Improved Subjugate Demon could apply attack speed on it, putting point into Demonic Sacrifice and / or Soul Link could unlock new Felguard abilities such Demonic Frenzy (+5% AP with every attack, up to 50% - TBC / WOTLK ability) or Felstorm (Felguards Bladestorm). Improved Fire Stone could give Felguard ability to proc FireStone effect, etc.
Immolation Aura - is fine, does what it is supposed to do, maybe could have some buff / bonus effect from Demo talent(s) or Demo runes - like temporary buff to damage / tick speed / visuals every time you cast Demonic Grace, Vengeance or when Dance procs - something cool that works well with its purpose / lore.
Shadow Bolt Volley - when only one target is hit, should deal full damage or maybe even bonus damage (something like +10%), could even cause some cool effect like shooting three separate smaller shadow bolts vs single target, maybe even doing triple damage ticks (3 x 35-40% of normal damage vs single target).
Metamorphosis - should be treated as core demonology rune and talents that apply to demon pets should apply to it in some way and as such should scale with Demo Talents heavily.
Hypothetical examples:
Fel Stamina - increase total health in Demon Form by 3-15%.
Fel Intellect - give warlock +20-100% dodge scaling from his intellect (just like other classes scale from agility) while Meta is active.
Improved Imp - give searing pain little splash damage while in Meta (like demon form in WC3).
Improved Voidwalker - buff to shadow ward, WOTLK style sacrifice that would take 80/60/40 % of pets max HP instead of killing him.
Improved Succubus - aforementioned buff to demonic grace, +50/100/150% damage bonus to shadow cleave (at +150% it should be slightly better than searing pain but it still would have 6 sec CD, Nightfall would also be much more viable in this situation in Meta).
Improved Health Stone - +10/20 max (or missing) HP restored on use or scaling with spell power while in Meta.
Improved Health Funnel - heal your pet for % of your damage while Meta is active, or maybe share some of your heals / self heals, or make it like master channeler while in Meta.
Fel Domination - recharges 3 times faster while in Meta.
Master Summoner - reduces mana cost of Meta by 25/50%.
Unholy Power - gain AP equal to 20-100% of your SP, add your auto attack to your Shadow Cleave damage, or increase threat or damage or make charge usable in combat and increase the stun duration while in Meta.
Demonic Sacrifice - maybe alternative effect while Meta is active, instead of killing the pet warlock is gonna take big chunk of its health in order to get some effect - heal, physical bubble, magic bubble, general damage bubble, spell reflect on next incoming spell - something tanking related, perhaps each pet should give different effect.
Improve Subjugate Demon - reduce duration of general crowd control effects while in demon form by +5/10/15/20/25% and demon specific CC by additional +10/20/30/40/50%.
Improved Firestone - make the proc scale with SP something like 5/10% or 10/20% and make the proc happen with spell damage as well.
Master Demonologist - remove the reduction to threat from Felguard while Meta is active, maybe replace it with increased threat instead? Reduce the cooldown of Vengeance or increase its duration and or HP buff for example.
Improved Spellstone - change it to consumable with multiple charges and 60-30 sec CD, or alternatively make it not disappear when you use it. Give it some love.
Soul Link - When Meta is active your Vengeance gains bonus affect something along the lines of WOTLK Meta, increased damage, huge temporary buff to power of immolation aura, turn charge into intercept, instant Howl of Terror, healing from Death Coil tripled, increase to healing received or self healing. Something cool that work well for Tanking and PVP bruiser.
