I was rooting around the fire emblem 3h wiki and I think I found potential Black Eagles route story spoilers... just bringing it to everyone's attention...
From what I read it seemed pretty solid, so black eagle people are in for a good story
Of course if the wiki is true, I have no way of telling because I didn't read Thani's leaks...
So PSA, if you don't want to get spoiled on post-timeskip plot stuff, don't look up Edelgard or the Adrestian Empire!
I skimmed the Faerghus/Dimitri and Leicester Alliance wikis and there's nothing too spoiler-ific there, just basic information (On Faerghus it has plot details about Cornelia and some lore behind her but that's it)