A fairly big non-spoiler is that we know what happens to The Empire. Something like 800 years after TOR is the Darth Bane era. By that point there isn't a unified Sith any longer and the pure bloods are extinct. From the books we more or less how it happens too. There isn't some big climactic victory for the Republic, the nature of the Sith is their undoing. They fracture into a dozen different Sith Orders who all turn on each other and give the Jedi a chance to get their act together. The Dread Masters are the beginning of the end of the Empire.
It will still be fairly cool to see how it all goes down though.
You completely skipped over Darth Ruin and his Empire,which comes before the Brotherhood of Darkness and then Bane and is thought to be built from pockets of Sith that survived from this era along with his Fallen Jedi.
All we know from the lore is that this version of the Sith Empire ceased to be,they could have destroyed themselves, they could have been beaten by the Republic, they could tire of war and joined the Republic,hell they could have won the war at some great cost(like a majority of the Purebloods) and put on the face of the Republic. The point is we don't know and it allows for freedom in the lore for the writers.
The Dread Masters are one of many major threats and it's pretty tough to point to them as the begining of the end. The Hutt Cartel is beginning to make a power play, the Star Cabal is still around, Malgus's apprentice is running around probably a bit peeved and we still have the Emperor drama.