Yavin IV with the tombs and Massassi survivors would be fun along with darkside cultists attached to Exar Kun.
Dantooine is mostly a farm planet with a ruined jedi academy and Rakatan ruins but it has potential.
Onderon/Dxun are a great idea too. I personally think it would be a mistake to have one without the other. Onderon with its Republic ties, civilization, and culture with a government that has been controlled directly or manipulated by Freedon Nadd's dark side cultists. Dxun with its savage jungles full of dangerous creatures, outcasts, criminals, sith tombs, and mandalorian training camps (heck some serious mandalorian stuff makes sense here given the connection to Mandalore the Preserver). An expansion here would also be a great place to introduce flying mounts (basilisk war droids and Dxun flying beasts).
Manaan could be an interesting world (kolto war anyone?) but I'd like to see something done for underwater if they added it. The Manaan also had started their own force using tradition as well so having both jedi and sith try to gain their allegiance makes sense.
Ziost is wide open to do almost anything with. By the time of the movies its a graveyard sith world filled with fallen lord's fortresses, tombs, and dark side experiments. With this being 3000+ years before that, who knows what Emperor Wannabe is trying to seize power from here? (hey it became a wasteland somehow)