As in, standard or operations styled maps that takes place in Sefoni’s mind realm. One possible premise being that you’re helping Sefoni defend against a psychic assault by a powerful, unknown force. This would be an excellent opportunity for creative mixing of multiple sectors and non-euclidean geometry, as well as continue the “Mortis Operator” story thread that’s been unspooled with Mortis Trials.
To illustrate, consider the following example:
We start in the Theatre, but Sefoni’s missing and the roof’s been torn off. Flashes of blue and green scatter the sky above as a psychic storm rages. We seek shelter in a new egress only to emerge in the ruins of Hab Dreyko. But it’s different now, having been interpreted through the vile mind of the enemy. Instead of being full of gross goop it’s halls are now infested with verdant overgrowth and vibrant plant-life, winding its way towards a beautiful willow.
We burn the tree.
This opens an escape into The Hourglass, its sands now fields of grass and flowers, showing what the enemy “thinks” the future of Atoma will be as a part of nurgles garden. We descend the cliffside structures down into The Torrent. Glorious waterfalls turn into rivers of bodies as we’re confronted with a clash of horrific reality and idyllic fantasy, eventually terminating in what remains of The Meat Grinder for a climactic showdown where we help Sefoni wrestle back control and give the intruder the boot.
We could give the enemy units flower crowns. Think about it.
(You could also reuse nature assets from VT2. I won’t tell, it can be our little secret)
Other Mortis Opportunities could be infiltrating an enemy’s mind to try and gain intel. Or Assaulting an enemy psyker’s mind palace to terminate the threat from within, literally. I don’t know about the lore logistics here, but if written properly I feel you could get a full 3 level storyline out of this concept.