It would be great to have one time use coupons. But not just a single use and the code cant be used, but single use per customer.
Also codes available only for new customers for their first order only. There is always the possibility a customer will order under new name and details but that isn't a huge concern.
Also codes only for customers who have spent a total certain amount on all their previous orders. But that comes into trouble with people who have returned or cancelled orders, the amounts shown they have spent becomes incorrect.
Also free shipping discount codes that work how they used to, spend a certain amount and recieve free shipping. But also have further control and add a limit to how much can be ordered and receive free shipping, example free shipping on orders over $100 and up to $300.
Also it would be nice if the dashboard for orders showed prices for products both with and without the discount code applied. So if an item needs refunding it can be refunded easily and correctly at the discounted price and not the full price. Having to do the math for that manually can be quite a pain and would be just more efficient if that was shown on the order page.
Also store credit would be great if it was separated from discount codes, so customers could basically have a real account with a certain credit amount and then able to use discount codes. Customer get quite cranky using a store credit code and then finding out they cant also use a discount code at the same time, and frankly I do not blame them.
Just a few ideas to think about Shopify, these are not new ideas either, I have seen pretty much all these mentioned in this forum for some years now.
Discount codes certainly need a lot of improvement and greater control for us Shop Owners.