Improved Natural Attack
Improved Natural Attack is a Feat that will increase the damage to 1d6. It's not a great return on investment, but it does exactly what you want. You won't qualify for it until level 8+ because it requires +4 BAB or higher.
Note that this is a "Monster Feat"; usually, this isn't a problem because they're not officially restricted to player characters, but some GM's might do so anyway.
Other Feats
- Rending Claws gives an extra 1d6 damage to your second successful claw attack in a round.
- Possessed Hand provides +1 insight to hit and damage at the cost of -2 to Concentration checks.
- Mother's Gift, if you are a changeling (which fits the class archetype), provides an untyped +1 to hit and damage with claws.
- Eldritch Claws causes your claws to overcome DR/magic and DR/silver.
Typical damage increases
Besides the obvious handwraps of mighty blows and general party buffs (eg bardic courage, etc), a common way to boost your own damage is increasing the size and effective size of your attack. You can only benefit from one of each type of effect at a time. Here are the most common options for those:
- Enlarge Person (witch 1 etc.), Steal Size (witch 2, etc.) Legendary Proportions (witch 7, etc.), and others increase you and your claws' actual size by 1 or more steps.
- Strong Jaw (druid/hunter/ranger), despite its name, increases the effective size of any natural attack by 2 die steps (1d4 becomes 1d8; 1d6 becomes 1d10).
Combining both effects will grant you 2d6 damage (2d8 with Improved Natural Attack) on your claws (and may give you a greater size bonus to Strength than your Hunched Muscle feature, at some levels).