A second raid to go along with Coil would be a great Idea, and I can think of a setting that would be perfect without having to introduce new lore last minute.

Praetoreum Ruins.

The unleashing of energy caused by the Ultima Weapon being destroyed has imbued Garlean creations with even greater power than their former master had ever hoped, and they are breaking out of thier bonds deep in the Ruins of the Garlean Stronghold.

No, this wouldnt be a "destroyed" version of the current Prae, but an entirely new area we have yet to explore.

This could also be a great place to Bring in some old but good mechanical bosses.