This comes from Studio Design Director Josh Sawyer, who teased the game on Twitter:
It remains to be seen whether this is a brand-new IP or the sequel to an existing series, but the folks at ResetEra have pointed out that Sawyer showed interest in directing a historical RPG on his blog back in 2016:
"I need to devote my attention to the project I’m currently directing, but it is my intention that the next project I direct (after this one) will be a historical RPG. It’s something I’ve been talking to Feargus [Urquhart, Obsidian CEO] every once in a while for the past couple of years and he’s been supportive."
What do you think this Obsidian project might be? Share your predictions in the comments.