Much progress has been made since The Big List of Possibilities #1466 was created 18+ months ago! Now as we head toward an actual 3.1 release, it is time to collect together all of improvements that may not be backwards compatible. The following is a list of the issues in scope for 3.1, and they should be considered launch blockers until each related PR has been approved and merged into 3.1.0-dev.
Nb: this is not an exhaustive changelog for v3.1.0.
As a follow-on to #1977, there needs to be some decision over how $schema should be used in the context of OAS. See issue #2017 which uses PR #2016 (starting point for a schema for OAS 3.1) as an example.