It has been known since before that Beamdog is working on a game tentatively called Adventure Y. Now they have confirmed on their blog that this particular game is in fact a new Baldur's Gate title. It's no sequel though, but rather intended to bridge the gap story wise between Baldur's Gate: Enhanced Edition and Baldur's Gate II: Enhanced Edition.
"This year, the eagerly anticipated Adventure Y will finally navigate the dark underground caverns of development and venture out into the Sword Coast. 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. Keep an eye on the Beamblog in late spring for announcements regarding the content and release date of Adventure Y."
Beamdog also reveals some thoughts they have regarding a true sequel with Baldur's Gate III, and that it is in fact something they have considered:
"We got a lot of questions about Dungeons & Dragons fifth edition and a potential Baldur's Gate III on our reddit AMA. We'd be lying if we said we never think about doing a Baldur's Gate III game, so we won't say that. But we will say there's nothing in the works yet regarding BGIII or a 5th edition game."