Ossus. OH GOD OSSUS. Perhaps a pvp zone for Jedi and Sith to fight it out over the ruins of the Great Library. That would be a pretty cool zone. A race against time as the planet suffers massive, critical seismic activity and lethal radiation from the ruined star threatens to bathe the planet in deadly light. Mandalorian hunters stalk the ruins, waiting for the ultimate prey - Jedi and Sith. Imperial Agents rush to recover ancient secrets, deemed too deadly and dangerous even for the Sith to pursue. And the Jedi Consular must discover the truth behind the Jedi and the First Great Schism - and whether the Jedi today are truly the descendants of Dark or of Light.
As much as I'd like to see Dathomir, it would seriously re-write the canon to have Jedi and Sith fighting during this period on the Planet. However, it would be really interesting to see Hapes. During this period the Cluster is run by pirates, if I'm not mistaken, and it's a period where the Jedi free the sector. That has serious potential for story quests, but it seems to work only as a Republic world. With that in mind they'd have to add a Sith world. With that in mind, I think maybe Mandalore or Ry'loth would be interesting worlds to add to the Sith list, and they would add a lot to the Imperial Agent and Bounty Hunter. Ry'loth would be interesting for Twi'lek Inquisitors.
On the other hand, searching Dxun or Yavin IV could be an equally valid option to add, especially for the Sith Inquisitor and Warrior story lines. Adventuring through the ruins of older Sith civilizations could definitely work well, and it would provide an opportunity for ample betrayal and intrigue among the Sith. And backstabbing is always fun.