Last time I made a thread about this topic, I got told by people with way more lore knowledge that it’s 3. Tertium being one of it and the others just being sub-clusters of it.
We know that there are 3 (Confirmed by Hedge) or more Hives on the planet. And isn’t that Hive cluster just Tertium ? With the 2 Main Spires and then the secondary clusters

I do expect the quarantined city to come into play at some point. My guess is that it will be our version of the Chaos Wastes with extended rogue-lite expeditions into it.
That would be actually quite cool. Chaos Wastes is arguably the best game mode of VT2, even in it’s post-nerfed state.
I’d settle for really interesting levels with new gimmicks or environments, too.