I would really like to see an integration with Mailchimp, so that a checkbox is added to the checkout window for marketing email opt-in. A bonus to this would be if the message displayed next to that checkbox was customizable (so you could say "I would like to stay in touch!" or "Please send me your newsletter" or anything of that sort), with a default suggested message.
Having this integration would be immensely helpful with donor stewardship for organizations (like mine) that don't have teams of development officers to update our data and communicate with donors at will.