So, tonight's new music was...unexpected.
If anything, this shows that while the project started at the tail end of 2015, they did leave some room for maneuverability to bring in stuff from games that released a ways into development. Does it mean we'll see one of Heroes' OCs get in as a newcomer? Probably not--it was still one of Nintendo's first forays into mobile gaming, and would've been just as risky as adding a newcomer from an IP that hadn't been revealed yet. Sakurai's always taken a "wait and see" approach to situations like these.
However...we could see some other secondary content come about from this. Trophies of Alfonse and Sharena? More Fire Emblem ATs? Maybe even another echo fighter, based on the rankings of the first Choose Your Legends poll...