This makes me wonder if perhaps there might be other Old Gods poised to move in on Azeroth who couldn’t do so previously because Yogg-Saron, C’thun and N’zoth were still entrenched there. Old Gods whose whispers are only now being heard by those on Azeroth attuned to the Void with the voices of the Big Four are all finally silenced.
Like maybe any newcomers’ influence would have been subsumed by the “native” Old Gods’ preexisting tendrils of corruption throughout the planet, but now that they’re all dead, it’s open season for whatever Old God pantheon can get there first and try to take their place.
Who knows; maybe Xal’atath was some sort of minion sent by a competing group of Old Gods to help facilitate taking down the ones currently infecting Azeroth so her masters could move in once they’re gone. Her conversation with N’zoth certainly didn’t sound like she was on Team Azeroth’s Old Gods; it sounded more like potentially a bargain being struck between N’zoth and Xal’atath on her own mission for herself or on behalf of someone or something else.