Tried posting this a while back but got little to no response, hopefully now those who want more raids will have more ideas.

Anyway, on to my suggestion:

Praetoreuim Ruins:
An unexplored section of the now abandoned pretoreium where the Garleans kept all their mechanical and biological experiments, and with some help from those dastardly Ascians they are awake, angry, and exponentially stronger than the Garleans could have ever hoped.

Note: this is not a Praetoreium hard mode, this would be a completely new section.

thoughts/ideas?