So this is my theory about how we could see the Sith return in the future. I believe Dath Bane's line was just one line of Sith. I think it's more than likely that 1000 years ago other sith survived and went into hiding. I mean there were many of them at one point and the sith are far more tenacious and prone to secrecy than Jedi. Given that it's very possible some Sith went into hiding and continued their lines in secret unbeknownst to Bane or his line.

Heck the Clone Wars shows how different lines can even split from one. We saw Ventress and Savage Oppress. At any point in the last 1000 years a sith could have thought he killed an apprentice, and that apprentice could have survived and started a line of their own. Plus with the fall of the Jedi any sith in hiding had their pick of potential apprentices.

In hindsight Palpatine's actions made him easy to hid from. If more secretive and stealthier sith could sense him (he wasn't really hiding) then knowing his nature and motives it'd have been easy to avoid him.

I think a good twist would be for the Brotherhood of Darkness to still be around and to have been rebuilding it's strength in secret after all this time. They just have to say that a few members didn't die way back and hid in the shadows of the republic becoming a sort of secret society of Sith that watched the Bane line in secret.

That's just one thought to really there are many ways sith could make a comeback but I am partial to the idea that Bane's line wasn't the only one out there.