1. Quinlan Vos
Fallen Order will need established Jedi. Obi-Wan Kenobi is practising his hobo skills on Tatooine. Yoda is in exile on Dagobah. Kanan and Ahsoka we’ve mentioned, and most others are dead. Yes, even the librarian.
What Respawn needs, in terms of narrative freedom, is an established Jedi character who survived Order 66. One whose fate is currently unknown, including what they got up to following Episode III. Their own version of Saw Gerrera or Captain Rex, a character fans know but the developers can tell their own tale with - even killing them if necessary.
Quinlan Vos is the answer to Respawn’s prayers. A more fun and rogue-ish Jedi Master, Vos first appeared in comics but got legitimized in Revenge of the Sith. Vos made a full appearance in Clone Wars but hasn’t appeared since - but a recent issue of the Darth Vader comic noted that he was still alive.
Vos is just what Jedi: Fallen Order needs: a fan favourite character, a more unconventional Han Solo-like Jedi Master, and a guy whose story needs to be told - preferably with a player character along for the ride.
You can have that idea for free, Respawn.