Beamdog has confirmed that the mysterious "Adventure Y" is in fact a new Baldur's Gate game. It won't be a sequel, however, but will instead take place in the gap between the Enhanced Editions of the original game and Baldur's Gate 2: Shadows of Amn.
"This year, the eagerly anticipated Adventure Y will finally navigate the dark underground caverns of development and venture out into the Sword Coast," the studio wrote on its Beamblog. "We’ve mentioned before that Adventure Y will be an addition to the Baldur’s Gate line using the Infinity engine, bridging the gap between Baldur’s Gate: Enhanced Edition and Baldur’s Gate II: Enhanced Edition."
That prior mention was presumably the one made way back in mid-2012, when Beamdog boss Trent Oster tweeted, "Keep your ears peeled for Adventure Y, which is intimately linked to the Bhallspawn [sic] storyline." In a separate tweet, he described it as "an optional storyline piece in the Bhallspawn [sic] series."
Beamdog said the official announcement of the game—title, release date, that sort of thing—won't be coming until late spring, and that the release will be "something bigger than we've ever done before." As for a potential Baldur's Gate 3, that's not happening—at least, not yet. "We’d be lying if we said we never think about doing a Baldur’s Gate III game, so we won’t say that," the studio wrote. "But we will say there’s nothing in the works yet regarding BGIII or a 5th edition game."