The existence of a fourth route, not a spoiler. How to get there, definitely is:
Each houses second half involves a war with the empire, or waging war as the empire. For Golden Deer it's called Verdant Wind and for Blue Lions it's called Azure Moon. If you pick one of those two houses, it's straight forward. However, if you choose the Black Eagles at the beginning, there is a choice prior to the timeskip where you make a decision between Edelgard (Crimson Flower) and the church (Silver Snow). I rolled with Edelgard the first time, and I think I'll save Silver Snow 'til after I've done the three houses in a normal way.
I should also say, I felt really bad about what I was doing with Edelgard, even if I thought Rhea was a total psychopath from early on. You really do see a difference between what a church ought to be for its people, and what one demented leader can turn it into.
I don't play a ton of games, just get locked into one for a month or two, then move on. I'm ready for the Last of Us 2. Cyberpunk 2077 looks cool. I'm definitely checking out Bravely Default 2 considering how much time I put into the first (loooooove the job system). And then, Tales of Arise looks promising.