They aren't blank though, just badly written. You can get Xander threatening Corrin with death near end game if she was lying about Garon being a monster even if the Corrin is married to him by that point. If they were worried about the possibility of the player marrying him, and writing him in a generic way in the main story to avoid clashing with it, that scene wouldn't exist in the first place.
Several characters change the way they reference Corrin if they marry him or her, even if that ends up clashing with the story scenes that never acknowledge those relationships.
It's obvious the main story writing wasn't held back by the intermission features, it just was bad in spite of that.
Main story is, independently, bad yes.
It isn't helped by having characters that have to have skin flint ties to each other so a vast variety of narratives work.
You used to have characters in tighter narrative groups and some main story content that fit into that.
Also, throwing the castle realm into the game, throwing the time travel into the game to get there to be a reward to the breeding at the expense of the world building and narrative....
Again, go back to FE4. You have generations. You have breeding. You also have a generational story. You even have character groups with distinct narratives.
That's a game that balances breeding simulation and narrative ambition.
Awakening and Fates...you have a crap story twisted to meet breeding sim requirements. You have narratively empty characters to meet breeding sim requirements.
...
If they do children again I want them a) to have the guts to actually make it a generational story b) to have the guts to put greater restriction on the breeding and go a more classical support route.