I've made a subclass that has the ability to create a soul shard when you kill a creature, you get a number of shards equal to your proficiency bonus. Is there a way to implement something like a Barbarian's rage that goes up as the characters proficiency bonus goes up?

At first it seemed like you wanted a feature to have a number of uses equal to PB, but then in your example you asked about a feature similar to Rage which has both a number of uses and a bonus. I mention that because uses (Rages per day) and bonuses (Rage Damage) are accomplished in different ways:

• The number of uses is accomplished by adding an Action to your class feature and then adding limited uses to that action. You can learn more about the process in the Subclasses & Races FAQ #4: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq).
• The bonus is represented using something called a “snippet,” to have the number scale according to PB you would use the {{proficiency}} snippet. You can learn more about snippets here: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/25930-using-snippet-codes-in-your-homebrew).

I hope that helps.