DLC isn't necessarily the same as an expansion, so I just assume that there are currently no plans for DLC or that they see DLC as a small update and instead are planning on something larger. I find it hard to believe that they intend for this to be the beginning and the end.
It's what Larian does: they don't "do" either DLC or expansions. The closest we've seen is Flames of Vengeance for Divinity 2 but the reality is that it was the final chapter that the publisher didn't give them time to finish.
