We usually add new releases as soon as they are released publically.
There are two issues with doing it earlier:
1) Compat data for a not-yet-released SAC may not be complete and change over time
2) We don't own the roadmap for Win 10 releases so we don't have the name of a new release until it comes out or shortly before.
We have considered adding a Windows next or similar setting to the selection of the target OS - if this is something that would make sense to you give us feedback in Windows Feedback Hub in the Windows Analytics for Enterprises - Upgrade Readiness for Enterprises category.