Slaughterhouse Nine and Nilbog:
I have even less to go on here than with the Endbringers. All I know besides the names is that they fall into the same category of “the monsters. The real dangerous motherfuckers, who are barely human any more, if at all.”
As such, I can only theorize based on the names, and at least two thirds of what I can think to draw from those are jokes. For instance, what I said when the names first showed up about Nilbog being goblins and/or people who want to get rid of bogs.
That said, the goal of the Slaughterhouse Nine is pretty clear: They want to throw a magical slab of beef into a volcano.
I’m thinking Upsilon’s goal might be to make an army of custom, mass producable parahumans. Newter and Gregor, among other amnesiac test subjects, are technically successes, but probably don’t quite fit what Upsilon or their leader/customer was looking for, causing them to get mindwiped and dumped.
Obviously the idea of such an army is terrifying and whoever is behind it probably has grand designs.
I’ve been pretty clear on this front, I think – I have a theory that at some point the extreme security of the Birdcage will be breached, causing a mass escape. That’s just kind of what tends to happen with fictional prisons that are supposed to be inescapable.
Hell, just take a look at Harry Potter – we learn of Azkabans supposed inescapability in a book that runs on the premise that someone escaped, and a couple years later, there’s a mass breakout that doesn’t even involve the same method.
Coil’s plan succeeding could go one of two main ways: The Undersiders are happy, or the Undersiders are not happy. In the former case, I guess the plot would carry on much like it has already, somewhat episodically but with a clear continuity, as the Undersiders work alongside Coil to actually implement his changes. In the latter case, the Undersiders would realize they’d been tricked into creating something worse than what was already there, and Coil would become the final boss of the story as the Undersiders work to take down what they helped create.
Of course, that’s assuming Rachel comes around and they do actually join Coil in this. If they don’t, well… I guess the second option is more likely, just with less irony of the “we helped create this” type.