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.
Yeah. The closest they've come to an "expansion" is Flames of Vengeance, but strictly speaking it wasn't an add-on but the final chapter of Divinity 2, the publisher having forced them to release the game without it (and I recall it caused a great deal of confusion at the time because of the "...?!" nature of the apparent ending). It's more their style to release a revamped version of the game as a freebie, something they've done with Divinity 2 and the Original Sins and any extra content tends to get rolled into that.
Obviously it's not necessarily the case that just because they've done it that way, they'll always continue to do the same, but I'd expect some sort of announcement if DLC became something they were seriously considering.