The warlock is nice as written but lacks the level 20 bump given to all Paizo classes. Here is my attempt:
At level 20 the warlock becomes an evil outsider (or chaotic outsider). Resistances gained from the warlock class become immunities. Choose two other energy types, these become Resistance (10). The Fiendish Resilience class feature becomes usable a number of times each day equal to 1+Cha mod.
In General...
Change HD to d8, 4+int skill points and following skills:
Acrobatics, Bluff, Craft, Disguise, Fly, Intimidate, Knowledge: Arcana/Planes/Religion,
Profession, Sense Motive, Spellcraft, Use Magic Device
