A lot of the main flaws have been discussed, but I'm going to throw in a few small ideas.
1. More Radiant damage feats for non-divine characters.
There are currently two builds centered around radiant damage, but most of the great feats for radiant damage are tied down to Divine powers. Furthermore, Wellblades are stuck with a power that doesn't scale because it applies vulnerability to all enemies in the aura, but Warlocks have few ways to deal radiant damage in burst.
Court Hexblades and Infernal Hexblades get good support for their damage types, but Star and Wellblades are left with little beyond Starfire Womb and Silverly Glow.
2. Evolving Blades.
By this, I mean the pact blade itself could improve over time. This could be as simple as all Pact Blades gaining Brutal 1 at Paragon and Brutal 2 at Epic, or as complex as gaining weapon properties or powers specific to your pact.
3. Intellingent Blades.
I would love
to see a Hexblade pact where your patron is the Pact Blade itself. STEAL THIS IDEA, WIZARDS. D:<
4. Hexblade Themes.
Warlocks come by their powers in a way that's completely different than any other class, so having unique themes could be a good idea.
5. Rods made for Hexblades.
I think this would be the best way to distinguish normal Warlocks from Hexblades. Normal warlocks have rods that focus on their curse, so naturally having Rods that can be interchangable but build on your pact weapon is a way to not only distinguish the two classes, but to distinguish the pact blade from any ordinary magic weapon (because Pact Blades should
be better than an ordinary magic weapon, given that the entire class is built around it).
For example, if Rod A has the property "Necrotic attacks made with your Pact Blade are crits on a roll of 19-20" and Rod B has the property "When you deal necrotic damage through an attack with your Pact Blade, you gain a +1 bonus to AC", having both rods and being able to adapt the pact blade to the situation would make the Hexblade a much more interesting class strategically.
I don't know - I think Wizards knows
that the Hexblade class has a lot of cool potential given that they've released three entirely new builds since HotFK, but just releasing a bunch of builds doesn't make the class any more fun. I think having unique Hexblade rods was the best suggestion in this thread, but the trouble with that is you'd have to collect a bunch of rods to make it work. I dunno.