DDO Eberron looks like it's ripe for a Warlock from Complete Arcane. Small selection of key ability spells that do not cost spell points, but you only get them with the same progression with which a fighter gains feats.
Perhaps also they would have 6 second individual cool downs similar to that of the Monk, but with a reverse energy method. Instead of building up the way the Monk does, perhaps warlock's energy pool of sorts would drain when using abilities. Except for Eldritch blast which would use none of their energy pool, but have a 6 second cool down.
The reason I say a 6 second cool down, is because that's how long a round in D&D is in time line. And thus this would similar how Warlocks can cast but one spell a round.
Or perhaps no energy reserve at all but a single universal 6 second cool down on all of their abilities. So that eldritch blast and every other ability would have a full round in between castings. This would actually be the most accurate way of implementing the warlock class and similar to it's table top counterpart. That way Warlocks can cast their spells an infinite number of times per day, but have a limited number available to them and can only cast one per round.
Warlock would offer slightly better protection and survivability than the Sorcerer or Wizard, with the ability to wear armor, d6 hit dice and built in damage reduction: but with fewer spells, a unified cool down to slow their spell flinging and A sacrifice in blasting ability for that increase to survivability.
The Warlock in DDO would ad a magic caster who's not quite so squishie, without it being the dominating powerhouse that Clerics tend to be. The Warlock would also be a magic caster that one could solo with more easily due to the increase in survivability.
It would make a nice stop-gap niche in between the uber blaster Warlock/Sorcerer squishies and the good luck killing me favored Soul/Clerics powerhouses. I'm already looking through the warlock evocation list, and I'm picking out ones that either already -have- a wiz/sorc spell in the game, as many warlock invocations mimic them. Or Invocations which would be easy to implement. I'll also list what D&D book each evocation is out of, for clarification.
The choices I made are not only easily compatible with the game, and easily within it's script structures, by using mechanics already present, but they also represent a nice range of abilities. I'll also post a codex so that you can see the book references and know what page the invocations are on in said books.
Name: Description. Book, page.
Codex:
Cityscape, (Ci)
Complete Arcane, (CAr)
Complete Mage, (CM)
Dragon Magic, (DM)
Least Invocations
All-Seeing Eyes: As comprehend languages on written
material, bonus on Search and Spot checks. CM, p123. (Adjust to just search and spot)
Beguiling Influence: Gain bonus on Bluff, Diplomacy,
and Intimidate checks. CAr, p132
Breath of the Night: Create a fog cloud as the spell. CAr, p132.
Call of the Beast: Speak with animals and influence their
behavior. CM, p123. (Same as Ranger)
Dark One’s Own Luck: Gain a luck bonus on one type of
saves. CAr, p133. (Choose save when selecting this invocation)
Eldritch Glaive: Form eldritch blast into reach weapon.
dragon magic, p82.
Eldritch Spear: Blast range increases to 250 feet. CAr, p133.
Frightful Blast: Target must make Will save or become
shaken. CAr, p134.
Hideous Blow: Melee attack channels eldritch blast. CAr, p134.
Leaps and Bounds: Gain bonus on Balance, Jump, and
Tumble checks. CAr, p134.
Otherworldly Whispers: Gain bonus on Knowledge checks. CM, p124.
See the Unseen: Gain see invisibility as the spell and
darkvision. CAr, p135.
Sickening Blast: Target must make Fortitude save or become
sickened. CAr, p135.
Swimming the Styx: Gain swim speed and ability to breathe
water. CM, p124.
Lesser Invocations
Baneful Blast: Eldritch blast deals extra damage against
specified creature type. CM, p123. (choose type upon sellecting ability at lvl up)
Beshadowed Blast: Target must make Fortitude save or become
blind for 1 round. CAr, p132. (6 seconds)
Brimstone Blast: Blast deals fire damage and target must make
Reflex save or catch fire. CAr, p132.
Charm: Cause a single creature to regard you as a friend. CAr, p132.
Cold Comfort: You and nearby allies protected by endure elements. CM,
p123. (As Protection from Elements)
Curse of Despair: Curse one creature as the bestow curse
spell, or hinder their attacks. CAr, p132.
Deteriorating Blast: target of your Eldritch blast must succeed
on a fortude save or have his damage reduction reduced.
Dragon magic, p81.
Dread Seizure: target take suffers wracking pains, reducing
speed by half and inflicting penalty on ranged attacks. Fortitude
save ends. Dragon Magic, p82.
Eldritch Chain: Blast jumps from initial target to secondary
targets. CAr, p133.
Flee the Scene: Use short-range dimension door as the spell
, and leave behind a major image. CAr, p134. (As Teleport instead)
Hellrime Blast: Blast deals cold damage and target must make
Fortitude save or take –2 penalty to Dexterity. CAr, p134.
Ignore the Pyre: gain resistance to one energy type. Dragon magic, p82. (Choose when choosing this Invocation)
Relentless Dispelling: As targeted dispel magic, with additional
targeted dispel magic the next turn. CM, p124.
The Dead Walk: Create undead as the animate dead spell. CAr, p133. (As Summon instead)
Voracious Dispelling: Use dispel magic as the spell,
causing damage to creatures whose effects are dispelled. CAr, p136.
Walk Unseen: Use invisibility (self only) as the spell. CAr, p136.
Witchwood Step: Walk on water and move through some obstacles
unimpeded. CM, p124.
Greater Invocations
Bewitching Blast: Target must make Will save or be confused
for 1 round. CAr, p132. (6 seconds)
Devil's Whispers: As suggestion, plus subject believes his
actions are his own idea. Ci, p68.
Devour Magic: Use targeted greater dispel magic with a touch
and gain temporary hit points based on the level of spells
successfully dispelled. CAr, p133.
Eldritch Cone: Blast takes the shape of a cone. CAr, p133.
Eldritch Line: focus eldritch blast into a line attack.
Reflex save for half damage. Dragon magic, p82.
Hindering Blast: Target of your eldritch blast must succeed
on a Will save or be slowed for 1 round. CM, p123. (6 seconds)
Noxious Blast: Target must make Fortitude save or be nauseated.
CAr, p135.
Painful Slumber of Ages: Creature falls asleep, takes damage
when awakened. CM, p123.
Penetrating Blast: gain +4 to overcome spell resistance.
Dragon magic, p82.
Repelling Blast: Target must make Refl x save or be knocked back.
CAr, p135. (Knocked down instead, as trip)
Vitriolic Blast: Blast ignores spell resistance and deals
acid damage for several rounds. CAr, p136.
Wall of Perilous Flame: Create a wall of fire as the spell,
but half the damage from the wall results from supernatural power.
CAr, p136. (as Wall of Fire)
Dark Invocations
Binding Blast: Target of your eldritch blast must make Will save
or be stunned for 1 round. CM, p123. (6 seconds)
Caster's Lament: Your touch can break enchantment, and you can
counterspell. CM, p123.
Dark Foresight: Use foresight as the spell, and communicate
telepathically with a close target of the effect. CAr, p133.
Eldritch Doom: Blast affects all enemies within 20 feet. CAr, p133.
Path of Shadow: Use shadow walk as the spell and speed up
natural healing. CAr, p135.
Retributive Invisibility: Use greater invisibility as the spell
(self only) that deals damage in a burst if dispelled. CAr, p135.
Steal Summoning: Take control of another caster's summoned monster.
CM, p124.