So, this door popped up on Reddit...

[Fenral]

That's clearly the entrance to....the VOID.

I mean even the "good" Allagans seem to agree that any problems caused by clones and voidgates can be solved easily: more clones and more voidgates.

[Magis]

If you look around Azys Lla, you'll find all sorts of hidden things. There are a few rooms full of broken experimentation vats.

[AstralKaos]

Stumbled into that passage leading there shortly after unlocking and queuing for Aetherochemical, it felt kind of eery and ominous... even still it does, weeks later.

Maybe the crude "sealing" of the door is a sign that it was done in a hurry, maybe before the place was abandoned (if it was in fact abandoned)?

[Balipu]

<Walks up to door> Fus Ro DAH!!!! <Door flys off of hinges>

Nothing to it.

Don't you mean Fu So Ya?

[Kotemon]

What might be inside?

Clearly it's;

Another door

[AdamuKun322]

It could also be the Eidolen's Realm where they sit, sleeping until called upon.

[Kalamari]

A chained door, some out of control/berserk robot boss I'd wager.