I would disagree with many of the declarations in this thread that Darth Nox would definitely rejoin the Empire.
I dont think Darth Nox would forsake the ways of the Sith, but I also don't think they would be happy returning to the Empire as Acina's lapdog either.
By the end of the vanilla story Darth Nox is certainly more powerful and accomplished than Acina, and despite losing your initial powerbase, you have rebuilt one in the form of the alliance.
I think it's much more likely that Darth Nox would maintain the alliance, in close partnership with the Empire - always seeking a moment of weakness in which they can usurp Acina and take their rightful place as Sith Emperor.