What we know so far: It is not one of their earlier cancelled projects being resurrected, they've been clear on that. "It's so secret it doesn't even have a codename" So it is not the Aliens:RPG or the dark-fantasy title bankrolled by Microsoft, it is also unlikely to be Wheel of Time.
As others have said before, it is also extremely unlikely to be a sequel to a property owned by someone else (Fallout 4, Jade Empire 2, Dungeon Siege 4...), because then the publishers would be the ones doing the announcement, not the development team.
Obsidian have a Kickstarter company profile now, and they have hinted that they want to do a spiritual sequel to Planescape:Torment. Not something that uses Sigil, The Nameless One or D&D rules, but something with a complex mature storyline in an original setting.
My anticipation feels stronger. I stand ready to update my journal.
The countdown quotes:
"What do the words mean?"The hidden messages in HTML source:
"Nothing. The Dirge of Eír Glanfath is sound without form, a lone voice crying out in mourning because it must."
"The book unread is unwritten. The reason we don't explain it is the reason we use it. Its power is in its mystery. That is the Leaden Key, in part, in whole. Is it clear?"
"Digging for truth buries the seeker."
"I recognize your pain. Your kind must learn it comes from straining against the turning of the world. It spins thus. You cannot stop it. One day, you will wake up. You will stop pulling. The pain will be gone. Until then, all your waking hours are suffering."
"This world wants to drag us down, it does. It made you a Watcher. No one asks for that. And the wight, that guilt they want to hang around my neck, you don't have to carry it one more step. You think they won't let you rest, but it's not up to them. It never was."
To tenderness, fellowship. To fellowship, vigilance. So bring we all to the Night Market. I am the Spindle of Esenath. Know me by my light and stand with me in darkness.The ". T . X X I T X" stuff slowly being revealed could be "Eternity".
Two centuries ago, your divine champion told the people of Dyrwood to grovel at his feet. If you've come on pilgrimage to the blasted crater that was our reply, Godhammer Citadel is that way.
In time, waves washing against the fingers of Geiran's Grasp will call them back into the sea. The difference between you and the sea is that the sea does not stop.
There is no sleep for the Watcher.