A lot of people focus on the spells when it comes to the Warlock, but I think the real look is at the Invocations.
At its core, the warlock is about at-will magic... magic on tap. That is a very cool concept, and some of the invocations capture that spirit. But so many just fall on their faces.
If you look at Devil's Sight, the reason its loved by Warlock players is partly for its power, but a lot of it is the simple fact you can do something no one else can do. Immediately flavor and uniqueness, perfect.
So many invocations fail at this. All of the "gain this spell" invocations, all they do is give you something any other spellcaster can just cast ... boring.
I think the answer is less on the spell side and more on the "giving warlocks more interesting invocations, and more slots for them".