I definitely agree on Camilla. I feel like she's a product of poor planning because Fate's writers often show glimmers of brilliance until it's squandered by an oppressive agenda, i.e., Camilla has to be yandere while other interesting development is second. She does have her moments though, like her A support with Niles.
Another big one has to be Corrin. There's a lot to fix here, and frankly I haven't even read them all. Azura (Conquest) is a big one that comes to mind, and here's what I would've liked to see:
Corrin and Azura
Canon supports between these two is focused almost entirely on Corrin, and I found that really frustrating. Reading through it again, it's like a hilariously bad chat role-play you might've done with your friends on KiK or something. The purpose of this support was to develop Corrin's true thoughts on siding with Nohr. Let's add to that.
Azura should peel away a bit of her past for Corrin -- perhaps on childhood isolation or shifting national identities since they both have that in common.
Develop both characters and nations by having each discuss their time in Nohr/Hoshido. Set the world as the moral grey it should've been.
Azura should help Corrin stand for the decisions they've made. Corrin should help Azura feel belonged and loved by the Nohrian royal family.
I also would've also liked to see Niles play out differently with Corrin. I think it would be interesting if Corrin actually liked Niles' slanderous tongue because it keeps them grounded as a person since it must be stressful to be chosen by the MacGuffin sword.
