Only confirmed units are LEphraim, GrimaF and WindLyn. Highly probably are the two remaining Bridal units (Ninian and Sanaki). Outside those, the rest is anyone's guess.
Smarter people across the web figured this out based on the fine print on the legendary banner advertisements at the good news ids if it is wrong, TONS of people will be wrong. But so far every L hero return has been legit predicted
Pokemon GO
Fire Emblem Heroes
Fate Grand Order
Dragonball Legends
Dragalia Lost
IV Calc
Heroes