ZFS feature flags have been mentioned earlier and now the code is now available from Delphix so that it can be integrated into illumos. With this in place new ZFS features can be implemented in a clean and compatible way, first out seems to be async destroy of datasets (feature flag com.delphix:async_destroy).
Hopefully we will see other new feature soon after this is in in place.
ZFS Feature Flags Presentation (PDF)
Feature flags webrev
Talks I have given
5 months ago