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
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.
Trial period in payment plans
Include trail period (1 week, 2 weeks, 1 month or free) options into the payment plans. So that it is visible to the paying client
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?
When payment links are created, payments received, recurring payments created, etc etc, call a user specified Webhook API for integration with other systems
Add a public API
Give accounts on our monthly plan the ability to programmatically manage their account. For example creating payment requests, payment plans and payment links.
Test Payment Link
I'd love the ability to create test links, plans, etc. Otherwise the website owner has no way of knowing what the entire payment flow looks like.