In the Super Smash Bros Ultimate Direct, it was revealed that in addition to the 74 (76) fighters already announced, we would also be getting 6 more fighters confirmed as DLC. The first of which being Piranha Plant, with the rest still unannounced. The way this announcement was worded in the Direct made it seem as though all fighters, including DLC, had been planned from the start, but Sakurai has just confirmed on Twitter that the 5 remaining DLC fighter have only just now been decided. This is big news since in the Direct, Sakurai specifically mentioned Rex not being playable since the roster was chosen before XC2 was announced, with his wording making it sound like he would even be out as DLC. But now the gates have opened to more recent video game characters having the chance to become playable fighters.
With this bit of information Sakurai also states that for the DLC fighters, Nintendo themselves picked which characters would become fighters, and Sakurai then decides if it can reasonably be done. Nothing major, but it is still interesting to get some inside info on how the roster is decided.
So go back and revise your predictions, as it seems like anyone is now possible.