Ask HN: I'm building an API. Suggestions?

Double edged question: 1) What is a good way to solicit advice for potential consumers of an API you might be building? 2) What endpoints should I support?

I'm building an API for FollowUp.cc, a service that lets you forward emails out to email addresses like 2days@, march10th@, etc... and sends you the email back at that time. At the moment, I'm adding the basics: creating reminders, finding all your reminders, finding old reminders, et al. I'd like to eventually add new functionality as well, like the "send this email later" feature Boomerang supports.

We're eventually going to have an API contest, but I first wanted to see if anybody out there has any suggestions on interesting features or endpoints to support.