I would prefer a full-blown sequel that evolves the Larian games in the same vein as the jumps from DOS1 to DOS2 to BG3, but if we're talking DLC...
-Something that expands the current game before the end. Extending the Underdark through Acts 2 and 3 could be very fun, and give us a Drow city-state maybe. Alternatively, fleshing out the city of Baldur's Gate to include the Upper City and some new districts, or let us explore the outskirts of the other side of the city. I would also take an extended Shadowfell Area in Act 2 to give a certain character more development, and elaborate on the Shar storyline. This area seems relevant to both Halsin and Shadowheart.
-Postgame content, which would likely take place either in the Astral Sea or Avernus. For all the mentions of Zariel, we never actually encounter her. The story with Vlaakith is also far from over. At the end of the game, two companion plot threads call for more story and point to activity either in the Astral Sea or Avernus. Also, there's no sun in the Astral Sea or in Avernus to accommodate a certain vampire.
-More levels.
-New Companion(s). Maybe another custom Origin like the Durge. (Dribbles, anyone?)
-More romance content.
-Friendships and Rivalries for companions.
-Flesh out camp companions.
-New content in areas in Acts that have been finished, and the option to return to them in the postgame. New threats to Druid Grove, or the Myconid Colony, or new expeditions to the Grymforge, or uncovering mysteries of the healing lands.
-New classes and subclasses. Maybe a new race.
