Consider a new faction that hasn’t had a real appearance in the campaign is in their prequel of sorts, it’s safe to say that they’ll be having their time to sign in another act.
Could it be that the neurotoxin is further refined so that it doesn't kill as fast as the current iteration? After all, Rasihidi did said he wanted to further refine it.
OR
The Foehn could use the neurotoxin as part for the "conscious slumber"? i.e. Used together with suspended animation using cyro-tech, since the Pacific Front specialized in cyro-tech.
As someone who’s worked in creating unique and interesting plot lines, I’d hope it’s not as simple as “just improve the toxin”, while there are masses of mind controlled forces all over the planet as resistance, the Epsilon’s main gimmick is the ability to mind control their opponents, throwing that out the window seems ridiculous. I’ll admit to being a touch biased against the Foehn, but they’re supposed defeat of the Epsilon would be much more interesting with the element of the Neurotoxin forcing them to have a limited amount of time to defeat the Epsilon, or them having to rebuild after the neurotoxin does it’s damage rather that finding some alternative to it.
I don’t see how the neurotoxin could be used in a “conscious slumber”, all it does is trick the common psychic, make yourself immune to mind control, and slowly kill you. As far as I’m convinced, it seems that cryo tech appears to have been abandoned by PF during the revolt if the Foehn arsenals are anything to work by.