Here are the only units left pretty much that can be Legendaries: If they go with these then all legendary candidates will be complete by mid next year: I used the heroes that were on the free 5* Leads giveaway banner that was awhile back. This is accurate on who IS thinks of as leads from there respective games.
Legendaries we Have
Mcorrin (Maybe not, we have female)
FByleth (Maybe only male or female)
Mbyleth (Maybe only male or female)
Mrobin (maybe not, we have female)
Note: If they have the Female version like Robin/Corrin, then the male might never be released since IS could think only one of the genders needs a legendary version (not too sure tho).