A New Hope is the original "de facto" Star Wars film. But if they were to follow it's plot, they wouldn't be able to have Luke as the sole Party Member. They would only be able to choose between both old Ben Obi-Wan Kenobi or Han Solo and Chewbacca, in terms of popularity, they could go with the latter. But honestly, the former makes more sense wielding a Lightsabre and everything. He would only be playable up to the point when the heroes leave the Death Star but after that, Sora, awakening to the powers of the Keyblade Glider like Ventus, Terra and Aqua. He could be able to fight alongside Luke during the Death Star run, that would be awesome to see. :]
If they included The Phantom Menace during Birth by Sleep, that would make more sense and an all no to The Force Awakens. The Prequel Trilogy makes up in storytelling where it lacks in quality whereas the Sequel Trilogy is the exact opposite. It has quality but lacks storytelling. I know I am being biased but the only good characters that seem to appear from the Sequels are only Poe Dameron, Commander Hux and BB-8.
If they decided to make an original story set during the Original Trilogy, they could potentially do so by setting it during the three year gap between both A New Hope and The Empire Strikes Back. Or during the one year gap between both Empire and Return of the Jedi. They could maybe even set it after the events of Jedi with the Rebellion rounding up the remaining forces of Stormtroopers and somehow, "Darth Vader" returns with the involvement of the Heartless. But if taking what they did with Pirates of the Caribbean in Kingdom Hearts III into consideration, odds are they'll start with A New Hope and continue directly onto Jedi. Since like Dead's Man Chest, Empire also ends "midway".
These are my guesses anyway.