A crossover with Ravenloft, Witchlight or Planescape couldn't be impossible in a future DLC. Even these could be a fabulous opportunity to promote other settings. For example the PCs travel toward a demiplane to fight against the temple of the elemental evil.
Of course we could see new companions, PC species and subclasses. I wonder about potential troubles with small-size humanoids, gnomes and halflings, with child-faces. Somebody could dare to try a romance between a gnome and a goliath (both adult age and facial traits), only to publish it in youtube.
I see Planescape and Spelljammer as more likely crossovers given future DLC will likely focus on higher levels.
Although it's possible that they go to Waterdeep, or Skuld (Capital of Mulhorand) , or Calimshan or Avernus, or Myth Nantar (Cool Sea Elf city in Inner Sea, whose mythal allows air breather to breath water), or Tymanther, etc...