Obsidian Entertainment has made a lot of games since it was founded in 2003, Kotor 2, South Park: The Stick of Truth, and Pillars of Eternity among them. In fact, thanks to a recent, helpful post from Obsidian itself, you can get the full list on Twitter.
There are some really good games on there, but the most interesting bit is down at the bottom: The "???," that is, suggesting that something new and secret is in the works. Sure, it could indicate nothing more than high hopes for the future, but when the studio was asked about its current undertakings earlier today, it was what you might call coy in response:
I've reached out to Obsidian to see if they're willing to share more, but in the meantime, remember this: Obsidian co-founder Chris Avellone, who left the company in 2015 but has remained very active in RPG development, recently maybe-sort-of teased something of his own on Facebook. Coincidence? Probably, yes, but if you're willing to run with the conspiracy theory long enough you end up with Avellone coming back to Obsidian to make a new Fallout game. That's a dream I'd like to hang onto.
Update: It's a pleasant dream, but Avellone has issued a statement saying that, whatever Obsidian is getting up to, he's not involved: "While there is no collaboration between Obsidian and I, I'm very interested to see what's next for them, as it's not a sequel to anything they've done before nor a franchise they've done before (it's not Fallout, Alpha Protocol, etc.)."