Depends on which Sith you want. The watered down Sith from Legacy aren't a good choice, except maybe Darth Talon.
But seriously, Lucas had HIS Sith. There could be other splinter groups out there that survived in hiding down through the millenia. Lucas said that his line of Sith started with Darth Bane. If that line of Sith ENDED with Palpatine's last clone, then maybe there are other groups that could rise to threaten the New Republic.