Follow-up of @jnothman comment.
I would like to propose to enforce milestones in pull requests.
That means:

• create a check that verify if the milestone is present
• modify the check on the change log to verify consistency with the milestone.

I believe it is helpful in anticipating breaking changes, and could give a more long-term vision on the library developments. It is also helpful for contributors, as they will have a better understanding of how much their pull request is awaited.
This approach is used for example by the Astropy project: no milestoned pull requests will still have their place anyway, they are long term feature implementations or code reformatting.