1) The problem → The current checkout modal is a pretty minimalistic form. It can't handle any upsells, doesn't reiterate any of the membership benefits, or have the ability to display any additional information.
2) Why is this important → As a marketer, I want to experiment with ways to increase conversions and to offer upsells.
3) What's your plan B → Try to recreate this with custom forms on a page, but it would still lack a lot of the functionality.
4) Possible solutions we could build for you → Create multiple checkout experiences: single column modal, two column modal, page on a subdomain, snippet to embed on a page (e.g. /checkout, /thanks, /upgrade)
Here are two examples of full-page checkouts (but can also be imagined as modals)