The ability to create coupon codes allowing customers to apply them at checkout.
Billing address collection
For high value, international payments billing address collection is essentially mandatory. It reduces "Do Not Honour" declines from the cardholders bank by ~50%. It's less important for domestic transactions, but banks are increasing looking at more metadata for international when deciding whether to accept or decline a transaction. You should have the ability to collect a billing address and pass this to Stripe correctly.
Ability to set currency when creating the payment link
When creating lots of payment links in different currencies you currently have to go back and forth between account settings to change currencies. This feature would streamline the process and make it really easy to work with multiple currencies.
Collect custom fields
Allow the ability to collect fields from customers when completing payments for one-time and recurring. Be able to send the data for that field via a URL query parameter?
Embed payment pages into websites
It would be useful if accounts had the option to embed payment pages like, Quick Pay, Payment links and Payment plans into their website so customers don't have to visit another site to complete their payment
Send emails from my email @companyname.com
Instead of sending payment link emails coming from Che at SuperPay, it would be nice to have notifications sent to clients from my company.
Pre-authorization for one-time payment links
Have the ability to pre-authorize payments then capture them at a later date manually.
Show supported payment methods
When checking out show the different payment types supported e.g Visa, Mastercard, etc.
Pass processing fee onto customers automatically
The ability to automatically pass the processing fee onto the amount the customers paying.