Darth Vader
Lets get this one out-of-the-way first. Most Star Wars fans know that between the events of Revenge of the Sith and A New Hope, Vader’s mission was to search the galaxy for and kill any remaining Jedi, so it would make sense that he and Cal would come face to face at some point.
The game creators have already stated Second Sister inquisitor will be one of the main antagonists of the games, as she has been heavily featured in the promotional material released to this point.
However, as the game progresses and the player becomes more powerful, it would make sense the player should come face to face with progressively harder opponents, and after Cal finally defeats the inquisitors in the game, the next logical step would be Darth Vader.
This wouldn’t just be fan service, as the inclusion of Vader would also fit the story’s narrative.
The protagonist Cal was a padawan when Order 66 was executed, and Anakin Skywalker/Darth Vader personally killed several Jedi and Younglings during the great Jedi purge. Cal likely was trained with and was friends with some of those Jedi and Younglings, and his master may have been killed by Vader himself.
If this is the case, than Cal would have a personal connection to Vader, who killed many of his friends and companions, in addition to betraying the Jedi Order, and he would have a personal stake in fighting him, which would hopefully lead to a climactic duel between the two.