I don't think it's meant as a tease - bosses as a default give a big XP bundle and drop (usually) 4 high-quality items, and it would take special scripting to make that not happen on the last one. Larian probably didn't have time to worry about that with everything else they were working to finish before release.
It wouldn't surprise me if there's an expansion (especially given how the sales look), but I wouldn't count on it also following Roderick and Scarlet. Their story is pretty much done by the end of the campaign.