IIRC, the new SW movies, 7, 8 and 9 are suppose to have Luke forming a new Jedi Order. If that is the case, perhaps they will bring Asoka Tano back to help and guide him. Think about it this way, she left the order in the end and is no longer a Jedi, which will explain how she was not in Ep. 3 and was not killed during Order 66. With Asoka technically becoming an official Jedi knight in the end of the CW series, it would be fitting, IMHO, instead of Luke depending on a bunch of Jedi ghosts helping Luke, to have an actual Jedi knight to help and guide her own Jedi Master's son, Luke, in re-creating a "better" Jedi order.