ActivityPub is a standards-track protocol for server-to-server federation, as well as client-to-server APIs at the W3C. https://www.w3.org/TR/activitypub/

My understanding is that this is something Mastodon is somewhat interested in already, but I'm filing an issue to have "somewhere" to track this.

Edit: here's a checklist for things to implement. I cut out client-to-server stuff, although most of it applies to both.

Does your application implement URLs for:

• Object id properties? (Section 3.2 Retrieving objects)
• Actor inboxes?
• Actor outboxes?
• Actor Following collections?
• Actor Followers collections?
• Actor Likes collections? (optional)
• Binary uploads? (Section 6 Uploading Media, optional)

Does your application handle:

• The special Public collection?
• Server-to-server Activity side effects, like Undo side effects?
• Linked Data Notifications RDF representations (very optional)? (Section 8.1 Delivery)

Do you:

• Properly support Actor simplified login naming? (Section 4)
• Deliver activities properly?
• Implement authentication and authorization checks for server-to-server interactions?
• Scrub Activities for cross-site scripting attacks?
• Have a plan for combating spam?