Hello,
Perhaps this is a good idea, but don’t you think that this should be directed to the developers of WC Marketplace? **The job of a payment gateway is simply to execute payments, not perform logic on what needs to be paid.** (This is again the separation of concerns principle that we have discussed before.) If you think that cashback is a useful feature, why is it only relevant to a cryptocurrency payment gateway? I would argue that it would be relevant to all types of payment, therefore it is the concern of the eshop, not its payment gateway. I am not saying this only because I am not able to devote time on it, but because it really is a feature that does not fit in a payment gateway. Ask yourself this: Would you ask this feature from a developer of a PayPal or Stripe gateway?
Happy holidays