Here is a blog for discussion and theories about the fan favorite villains of the series (and ironically three of the few if not only villains fans actually wish death on in the series): The Dazzlings.
Mainly about where they disappeared to and whether they will return.
Some say the Dazzlings could possibly return for revenge or redemption.
Some say the Dazzlings aged into dust without their pendants to keep them alive as they did for over a thousand years. (which of course would have been too gruesome to show onscreen and Twilight's comment would imply the opposite)
There is even a theory that, due to pony toys of the Dazzlings, the Dazzlings were once ponies until they found the pendants which either transformed them into sirens or actually contained the spirits of sirens which possessed them. And their continuing to TRY to sing their most recent song after losing that power was residual influence much like the effects the Alicorn Amulet had on Trixie.
So, here's a set of polls about the Dazzlings.