I also say the 6.7 Beta ISO would start with the actual 6.6 feature set. Allowed changes in the Final version:
- default values changes
- changes to UI appearance, behavior
- new (documented) features, compatible with existing ones
After 6.7 Final is released, it becomes the new “rolling release” and 6.6 will not receive any update.
If I remember correctly these are the same rules applied to the 6.6 release.