As it's presented in the base game (ignoring the whole XV universe, this is something I think many are failling to separate when talking about what needs to be added/expanded on FFXV as well as what's important for XV the game, not XV the universe) Niff IMO needs little focus outside of one character, Ravus. He's the only character (outside of Ardyn) to have a grudge againts him on a personal level for making her sister life deteriorate (if indirectly).
Loqui needs none, he's a one off villan that amounts to nothing in the overall plot of XV (like Ultros was in VI).
Caligo, MAYBE, as it is the only reason to focus on him would be to avenge Jarred, but little else.
Verstael and Iedola need no more, they are not important in any way to Noctis story (the former is important only to someone else and the later did almost nothing because Ardyn was the one behind everything).
Glauca also doesn't need focus because, the game never brings up nor focuses on who murdered Regis.
90% of the reason why people have a problem with the lack of development of these characters is because a lot of people don't like the Universe.
Iedolas gets the perfect amount of characterization in Kingsglaive, so does Ravus, Regis, and Luna (bar some inconsistent shit in the writing and what some of the characters do).
The problem with it, is that all of these characters get fucked over in the main game as a result, because it IS Nocts story, and everyone either had their entire story told through KG (Glauca, Clarus, and Nyx), most of their story told (Regis and Iedolas's rivalry was something advertised for years before FFXV even released, but their story continues ever so subtly in the main game, along with Regis's regal presence and how much he cares about his son) or had important things explained (Luna getting the Ring, Ravus getting rejected by the same ring, the Kingsglaive's fall) without really connecting it all back to the main game effectively.
If they just adapted Kingsglaive as a supplemental episode in game with Regis, Nyx and Luna as the main protagonists, and focused on Iedolas, Ravus and Glauca as the villains, as well as connect these 2 things better throughout the main game, it practically solves nearly ALL of the problems with Niflheim and the protagonists that needed more.
And I'm inclined to agree because I'd rather play a FF story than watch one. It's half the reason I hate Advent Children (aside from having a dumb story).
Edit: And the same thing goes for Brotherhood, but since it has nothing to do with the Empire for the most part there's no point in talking about that.
Tl;dr: I personally think expanded media sucks when it is mandatory and not supplementary to a characters story. Kingsglaive is necessary to understand fully almost all of the main plot's conflicts.
