Since time is obviously an issue, and making new spells takes up time, here are some suggestions for abilities that could potentially be added to the Warlock spell list, and keep them from becoming a very poor version of a wizard or sorcerer. For some of these, renaming them would be a good idea, but that's not really an issue.
Name: "Heatwave"
From?: Various "Salamander" enemies.
Reasoning: This aura increases vulnerability to fire. Fiend Pact Warlocks rejoice!
Name: "Earthgrab"
From?: Any earth elemental; dwarf racial tree.
Reasoning: It renders the enemy helpless and deals small amounts of bludgeoning damage. There were two similar spells in the 3.5 Warlock spell list, I believe, but they weren't as good.
Name: "Shadow Manipulation"
From?: Shadowdancer ED
Reasoning: This charms an enemy, then kills them when it's duration runs out. That's a very Warlock-ish thing to do to an enemy.
Name: "Shadow Form"
From?: Shadowdancer ED
Reasoning: It's very much like a PM undead toggle, and the strength damage on-hit will be useful.
Name: "Diversion"
From?: Level 18 Ninja Spy Core Ability
Reasoning: Well, Warlocks can't teleport out of danger in DDO, but this is the next best thing.
Name: "Mind Blast"
From?: Any Mindflayer enemy.
Reasoning: This would be most appropriate for a Great Old One Pact warlock, but really, it's an AOE Stun!
Name: "Elemental Weapons"
From?: Artificer spell list.
Reasoning: Warlocks are going to be in melee a lot, might as well give them something to enhance it a bit.
Name: "Kukan-Do"
From?: Shintao Monk enhancement tree.
Reasoning: Remember a spell called "Eyebite"? In 5th edition it gave you the option of stunning an enemy from a distance via your eyes. Might want to rename it, though.