It looks like ncsoft's attorney filed a trademark on Secrets of the Obscure in March using a shell company called Secret River Enterprises. It's the same attorney that did other GW2 trademarks. They also did not prefix the trademark with "GUILD WARS 2: " like the other expansions.
So I don't think it's a placeholder, unless they're really committed to fucking with us.
"Secret River" and the "Secrets of the Obscure" give me some River of Soul vibes...are we perhaps going to the Underworld? It's supposed to be under new [demonic] management now, right? And we just had to deal with a demon in Gyala Delve...