Same characters or different, I think both could work. For the same characters, more Harper shenanigans, perhaps this time vs. the Zhents or Cult of the Dragon. Zhents could hint at a deeper conspiracy from the 1st movie, Cult could run off the dragon appearances from the 1st film. Personally, I'd go for some sideways hints to Wave Echo Cave, Phandelver and/or Baldur's Gate.
Either way, having Xenk show up again as a recurring character would be something I would definitely like to see. And some sort of sidewards involvement from Szas - perhaps directing things from the background for revenge against foiling his plans in the first movie.