My main thing is that it feels like the warlock has to do a lot of work, and what is the point of selling your soul if you still have to work hard?
I figure the warlock ought to be a smart guy or girl (or whatever) who thinks it isn't fair that his/her/their brains don't get as much respect as some big fighter or barbarian's muscles, the rogues fancy footwork, or the bard's charm (I am going with warlock as an int caster).
The main feature of the warlock becomes otherworldly aspect. He/she/they can still have eldritch blast and hex and what not, but that is just stuff they do. Otherworldly aspect is what they want.
So, if the warlock chooses an archelemental patron, their aspect lets them use their int modifier for their strength modifier (and we can throw in some weapons proficiency, etc.) Charles Atlas, eat your heart out.
The archfey patron gives your aspect fey's grace (use your int modifier for your dex modifier, including for initiative). And we can throw in some proficiency with ranged weapons and light weapons.
The undead patron gives your aspect undead fortitude (int modifier for con modifier and a bunch of temporary hit points).
Do you see how many tentacles and eyes aberrations have? They ought to have really good perception scores, which means the GOO patron gives the warlock's aspect the ability to See the World as It Is (use int modifier for wisdom modifier and some psychic stuff).
Sure, some people might make deals with fiends to become some big, nasty monster, but most people would rather be loved or feared (and summon a big, nasty monster), so prepare to be the life of the party or the scariest guy in the room with Fiendish Presence (use int modifier for cha modifier and expertise in persuasion or intimidation).
The other big thing I would do is make the book, blade, and familiar into invocations. You have to pick one at 3rd level (or 1st if they make subclasses start then), but if you want, you can get all 3.
Minorly, I would make planar ally a mystic arcanum spell (adding fey and undead to the possibilities) with the understanding that warlocks always have to pay (there are some benefits from getting power from a god your worship, and the possibility of cheap labor is one of them).
I think fighters and rogues should get a 1/3rd caster based on the warlock (specifically the blade) to go with the subclasses based on the wizard (spell thief and eldritch knight). It would be easier to work the "I just want a frontline guy with a magic sword and some spells" that a lot of blade pact players seems to want (although why your warlock would want to risk his/her/their life on the frontline is a good question) then even the archelemental patron would do, and it would also take care of some of the warlock players who might be put off by the focus on otherworldly aspect.