By this time, though, Vvardenfell has been finished off by the moon over Vivec, and the surviving Dunmer would be in a weak position to maintain the practice in the surrounding provinces - particularly Black Marsh. I expect that large-scale slavery is gone. That said, I can see slavery in two forms:
1) Vampires. It can be argued that vampires still enslave people when turning them into cattle. I would expect that if vampires make it back into the game ( and I'm sure they will ) there will be slavery in that form.
2) Wizard enclaves. I don't know if the Telvanni will be back, but it is likely that some of them escaped to Skyrim, and brought their slaves with them.
Either way, I doubt the Twin Lamps will be necessary.