should they just open diadem as a regular area now?

[Canadane]

I still find it funny that the only content that you consider "FC content" is content that wasn't even exclusive to FCs at its initial release. You could queue up for Diadem 1.0 at the Ishgard airship landing. FC airships merely allowed groups to queue up for the hard difficulty, after unlocking it (while the rest were limited to easy/normal).

All they'd have to do is re-enable the Ishgard part, and add the unsync option (like all the other HW DF content).

You can still queue via Ishgard. It never went away.
Never has and never will be FC exclusive content.