It wouldn't surprise me if it's KOTOR-based. Not the same necessarily, but set in that time-period, putting a new spin on things. Also Revan has been a character that fans have wanted to see return for a long time now (and Disney has shown with Thrawn that they're not above bringing in popular Legends characters in new settings). Or it might revolve around Darth Bane/founding the rule of two perhaps? Personally, I'd love to FINALLY see a female Sith Lord in one of these films.