It's easy to assume it could eventually happen, but there's also the fact that Larian never released an expansion for any of their past titles so far.
That aside, at this point in time and with all the work left to do on the base game, I'm not sure if even they could answer the question with any degree of confidence.
On the other hand, there is question if any of ther "past titles so far" was so big franchise as Baldur's Gate. O_o
Know what i mean right? The options for DLC's is almost limitless in this case.