The main issue with this subclass was his ever increasing damage even when multiclassing out. To fix that, I thought it would've been much more meaningful to put a different scaling on the bonus damage that would only account for Warlock levels, and to rework the Improved Critical part of the curse to work only with melee weapon, as it is much more sensible for the spirit of the subclass. The martial weapon proficiency was removed as irrelevant and the Charisma substitution is limited to enhance the likelyhood of picking the blade boon (which should be the main choice for this subclass).

Starting at 1st level, you gain the ability to place a baleful curse on an enemy. As a bonus action, choose one creature you can see within 30 feet of you. The target is cursed for 1 minute. Until the curse ends, you gain the following benefits:

At 1st level, you gain proficiency with medium armor and shields . In addition, when attacking with a simple melee weapon or one you created with your blade pact boon and that lacks the two- handed property , you can use your Charisma modifier, instead of Strength or Dexterity, for the attack and damage rolls.

The Hexblade lets you choose from an expanded list of spells when you learn a warlock spell. The following spells are added to the warlock spell list for you.

I think that the Hexblade flavour was pretty bad, and the best way to improve it it's to change the nature of the Hexblades themselves from actual, physical weapons to Entities of great power that can manifest a part of themselves as blades.

The Raven Queen, following mysterious plans, purposefully created perturbances in the Shadowfell to spawn Hexblades; she is even rumored to have forged the first of their kind, while the others followed because of the destabilization of the planes. These particular Hexblades, along with their Warlocks followers, are ubidient tools she can use to manipulate events in the Material Plane to her inscrutable ends.

Fractures between the material plane and the Shadowfell, the Hexblades are not as much weapons as beings of incredible power and intelligence. Their aim is to expand themselves through dark rituals and offerings, free the multiverse from any boundaries and make it plunge into complete chaos and freedom; to do so, they manifest themselves in a multitude of sentient weapons which bear a fraction of the power of the original blade. Warlocks who aim great power and the coming of a new age often choose an Hexblade as their patron.

Shadow Hound (no changes)

Starting at 6th level, your shadow can split from you and transform into a hound of pure darkness. Most of the time, your shadow hound masquerades as your normal shadow. As a bonus action, you can command it to magically slip into the shadow of a creature you can see within 60 feet of you. While the shadow hound is merged in this manner, the target can’t gain the benefits of half cover or three-quarters cover against your attack rolls, and you know the distance and direction to the target even if it is hidden. The hound can’t be seen by anyone but you and those with truesight, and it is unaffected by light. The target has a vague feeling of dread while the hound is present. As a bonus action, you can command your shadow hound to return to you. It also automatically returns to you if you and the target are on different planes of existence, if you’re incapacitated, or if dispel magic, remove curse, or similar magic is used on the target.

Armor of Hexes

At 10th level, your hex grows more powerful. If the target cursed by your Hexblade’s Curse hits you with an attack roll, one time a round, you can roll a d6. On a 4 or higher, the attack instead misses you. On a 3 or lower, you take the hit but get to use this feature again for this round.

Why I like this change Armor of Hexes was just too good. With a lucky streak you could avoid all attempts to harm you without much interaction or interesting outcomes. This way, the Warlock has to choose which attacks he wants to attempt to avoid, so there is some sort of tactical thinking, while it can still be really good when using it to avoid some dangerous spell.

Master of Hexes

Starting at 14th level, you can use your Hexblade’s Curse again without resting, but when you apply it to a new target, the curse immediately ends on the previous target.

Eldritch Invocations

Curse Bringer

Prerequisite: The Hexblade patron, Pact of the Blade feature.

You can create a longsword forged from silver, with black runes etched into its blade, using your Pact of the Blade feature. If you reduce a target cursed by your Hexblade’s Curse to 0 hit points with this sword, you can immediately change the target of the curse to a different creature. This change doesn’t extend the curse’s duration. When you hit a creature with this weapon, you can expend a spell slot to deal an additional 2d8 slashing damage to the target, with an additional 1d8 damage per spell level after the 1st, and you can reduce the creature’s speed to 0 feet until the end of your next turn.