should they just open diadem as a regular area now?

[Nezerius]

I don't count the submarines or airships as FC content when it takes like a couple months of diligent work by a group of people to finish everything in it.

So you don't consider content tagged as FC content to be FC content. Okay.

Need more FC only accessible areas like the diadem.

Only if they can manage to keep it interesting enough for players to keep running it a month after release. Otherwise it'll be just like the Diadem, dead content.