Eldritch Metamagic [3.5, Feats]

[Admiral Squish]

Eldritch Empower:
Prerequisite: Eldritch Blast (3d6)
Benefit: You can reduce your eldritch blast’s damage by 2d6 to increase the damage dealt by your eldritch blast by 50%.

Eldritch Maximize:
Prerequisite: Eldritch Blast (4d6)
Benefit: You can reduce your eldritch blast’s damage by 3d6 to maximize the variable, numeric damage of your eldritch blast.

Eldritch Quicken:
Prerequisite: Eldritch Blast (5d6)
Benefit: You can reduce your eldritch blast’s damage by 4d6 to use it as a swift action instead of a standard action. You cannot use your eldritch blast more than once per round.

Eldritch Still:
Prerequisite: Eldritch Blast (2d6)
Benefit: You can reduce your eldritch blast’s damage by 1d6 to use it without somantic components.

Eldritch Widen:
Prerequisite: Eldritch Blast (4d6), Eldritch cone or Eldritch Doom eldritch shape invocations.
Benefit: You can reduce your eldritch blast’s damage by 3d6 to increase the area of your eldritch shape or eldritch cone by 100%.

Eldritch Knockback:
Prerequisite: Eldritch Blast (3d6)
Benefit: You can reduce your eldritch blast’s damage by 2d6 to knock back creatures struck by it. A creature that takes damage from your eldritch blast must make a reflex save or be pushed away from you in a straight line 5 feet, plus 5 feet for every 5 damage dealt by the blast. If the blast moves the target more than 10 feet, they take 1d6 points of damage for every 10 feet moved. If some object or creature prevents them from moving as far as the spell would force them to, they take 1d6 damage from striking the barrier (Plus any damage taken from moving normally) and stop moving. If the barrier is another creature, they too take 1d6 damage.

Eldritch Mercy:
Prerequisite: Eldritch Blast (2d6)
Benefit: You can reduce your eldritch blast’s damage by 1d6 to make your eldritch blast deal nonlethal damage.

Eldritch Twin:
Prerequisite: Eldritch Blast (5d6)
Benefit: You can reduce the damage of your eldritch blast by 4d6 to fire two blasts at the same time. The two eldritch blasts roll attack and damage separately and can be aimed at the same target or at different targets. You must apply the same eldritch shapes and eldritch essences to both blasts.

Eldritch Retribution:
Prerequisite: Eldritch Blast (2d6)
Benefit: You can reduce your eldritch blast’s damage by 1d6 to cast it at a retributive blast. A retributive blast has all the normal effects of an eldritch blast, including any eldritch essences applied. When dealt damage by a melee attack, you can choose to release the retributive blast, allowing it to instantly affect the attacker, rolling attack and damage normally. The retributive blast can only affect the attacker. In a retributive blast is not released within one hour of casting, it dissipates.

[Admiral Squish]

I've always been scratching my head a bit at how eldritch blast interact with metamagic feats, and while I was making a warlock/rogue character, it occurred to me that one could steal the ambush feat pattern and apply it to metamagic to get an easy-to-use solution for warlocks looking for some feats to add oomph to their blasting. So, here we are.

This is just a crude translation of the megamagic feats, with 1d6 eldritch damage=one spell level. I'm sure people more experienced with metamagic than I will be able to come in and help me hammer these into something more resembling usable shape.

[Rolep]

Eldritch blast already benefits from the meta-spell-like-ability line of feats. I'm not a good enough player to do the maths to see how this would play out, but it's just something for you to bear in mind.

[Admiral Squish]

Well, if it does, that doesn't seem clear to me at all. The metamagic spell-like-ability feats have a limitation on what level of spell they can affect, that's usually one or two levels behind your current level. And if I'm reading eldritch blast right, it's always a SLA with a spell level equal to one-half your warlock level. So, in theory, the two never meet. Of course, I could totally be reading it wrong, but I like to think my version's a lot simpler and less finniky.

[Rolep]

Well, if it does, that doesn't seem clear to me at all. The metamagic spell-like-ability feats have a limitation on what level of spell they can affect, that's usually one or two levels behind your current level. And if I'm reading eldritch blast right, it's always a SLA with a spell level equal to one-half your warlock level. So, in theory, the two never meet. Of course, I could totally be reading it wrong, but I like to think my version's a lot simpler and less finniky.
No, the level of EB is equal to the highest level essense or blast shape applied to it (if none are applied, it is elvel 1). Yes, your version is simpler, but I'm worried that the combination of the two might have OP side-effects.

[Zaydos]

As a prefatory note the errata to Complete Arcane changed EB to a 1st level SLA unless a blast shape or eldritch essence was applied to it.

Looking over these.
Empower: Strictly worse until you get 6 dice of damage (possible at 7th with Greater Chasuble of Fell power although unlikely). Not really worth a feat until you have at least 8 (possible at 11th with said item where it is affordable) and at that point it's simply +1d6 damage. Possibly too strong on a Hellfire Glaivelock (approximately +5d6 damage/attack at 20th), otherwise the damage boost is kind of needed.

Maximize: Similar in that it's really not worth it until rather late (specifically you need 8d6 for it to show any improvement (2 points) and it is eclipsed by Empower until 11 dice (20th level without hellfire but with item). Hellfire makes it better but outside of glaivelock I don't see taking this.

Quicken: Conversely this is simply +Xd6 damage each round. You wouldn't hellfire this (unless you have Naberius or Strongheart Vest) but even without it this is +6d6 damage, possibly in an area, with a save or debuff each round (at 20th). It would definitely be my Lv 9 feat choice (+2d6 or +3d6 damage per round and extra save versus Blindness if I went that route which I might not).

Eldritch Still: Wouldn't take. It has uses, but really you could probably remove the damage debuff and it would be still balanced.

Eldritch Widen: Might be worth it, I guess, maybe for a debuff build (something like this and Eldritch Quicken on an eldritch doom enhanced with the stunning effect from Complete Mage to force a 40-ft area of enemies to save or lose a turn each round).

Eldritch Knockback: Can't comment.

Eldritch Mercy: Like Eldritch Still you should probably drop the die cost altogether.

Eldritch Twin: Math time again! My first inclination is to say this isn't worthwhile until you have at least 8 dice of damage, but that's not true, as it forces a 2nd save. Combine with Eldritch Quicken and you can force 3 multitarget save or sucks as soon as you have both feats (as you can use items to qualify if you can manage to get a Greater Chasuble by Lv 6 you could do it as early as 9th, most likely you'll be waiting till 12). Damage-wise this only becomes worthwhile at 20th without hellfire or chasuble. With chasuble it becomes worthwhile possibly at 14th (+2d6 damage/round) and is eventually actually pretty good (+4d6 damage per round; allowing for a whopping 21d6 damage/round with Eldritch Quicken). Again it's hellfire which makes this get scary as it boosts it by another 6d6 (33d6-39d6 eventually). Even then it's not enough to outpace Hellfire Glaive (54d6). Unless it can be combined with Edritch Glaive to give you two full attacks with Eldritch Glaive (in which case you're talking 84d6) but I don't think it can.

Eldritch Retribution: Nice, a free action blast 1/encounter (in effect). The -1d6 hurts at low levels but is mostly unnoticeable at higher ones.

On stacking with meta-SLA feats.

Nothing that I've said really changes (due to the standard rules on stacking empower and maximize). Twinned sees the biggest change (by nova-ing you could do 168 damage in a single round fairly easily but again nothing to hellfire glaive).

Overall some neat options, Quickened is a bit of a must have, and some are under powered but with standard warlock needing a damage boost everything looks fine except when mixing with the two official tacked on damage fixes (hellfire warlock and eldritch glaive) at that point maximize might be a problem (hellfire glaive gets up to ~250 damage/round but requires access to swift action movement).

[Admiral Squish]

Now THAT is some lovely commentary!
It's great that you actually went through and did all the math and figured out how useful each one would be. The only one that you didn't quite hit on the head is quicken which includes the line 'you cannot use your eldritch blast more than once in a round'. The idea was to make it so you could quicken your blast and do other things with your invocations and whatnot, it's not just a flat addition to your damage output. Makes it so you can keep outputting (reduced) damage while you do other warlocky things like using hungry darkness or such.

Your commentary is quite thorough, but a lot of the feats seem rather weak by your estimation. If I went across the board and reduced the damage reduction by 1d6 on all the feats, do you think that would make them more appealing? Or would that, perhaps, be too much for some?