I'm still a long ways away from the finish line. To be clear about something, the Black Eagles story is actually multiple chapters shorter than the other 3 routes for some reason. I'll post a review of the game when I complete my Blue Lions run though. As a vague outline, I think this game has definite upsides and downsides. With respect to your point about the different paths for instance, I'm actually not entirely happy about including all the paths on the same file instead of selling them separately. The reason I feel this way is because all the paths are the same pre-timeskip, which makes 2/3 of the game tedious if you want to do another path. I actually preferred Fates because at least each route was a separate game, although even there maps were reused (though not entirely reused; the enemy placements were at least different on reused maps). If I have to make a simple choice between A.) Three paths for free, but all of them have recycled content or B.) Three paths, all of which cost money, but each of them are unique, I'll choose B. In the case of Three Houses, it feels like they wrote the pre-timeskip for Blue Lions and then retroactively applied it to Black Eagles and Golden Deer. There is no reason why Golden Deer should care about Ashe's adoptive father or Sylvain's brother. It simply does not make sense for them to experience the exact same conflicts as the Blue Lions. Nonetheless, this is what Three Houses gave us. I would have gladly paid an extra $20.00 or so for a fleshed out Golden Deer or Black Eagles pre-timeskip plot where they dealt with conflicts within the Alliance or Empire respectively. I like Fire Emblem enough to be willing to shell out the extra cash unless I'm severely burnt out on the gameplay. An easy counterpoint to the dichotomy I have presented is that you could say, "Well it is not mutually exclusive that they can offer multiple paths for free AND make each of them unique." I absolutely agree with this, but unfortunately that is not what we received with either Fates or Three Houses.
I'm not as concerned about storyline in Fire Emblem. It's why I like Fates even though I think the story and lore is utter trash. With respect to Three Houses, I think the story has some good points and bad points, but I love the lore. It has a lot of fine political details which I appreciate, including some sort of backstory for all the noble houses. I distinguish between story and lore in the following way: The story for me entails the main adventure and the trials and travails that the protagonist and other heroes experience. In all honesty, I never expect much from the story of a Fire Emblem game. I usually enjoy these games more for the supports that flesh out characters individually. The lore (by my definition) concerns character house or individual backstories or details about the world and its history.
If I do review the game, you will find that I do not even consider story as a point of interest in my review. This is not because I don't care about the story. It's because I don't value story much in games, or even at all. Gameplay is far more important to me. As far as the gameplay in Three Houses is concerned, it's somewhat middling by my estimation. I would consider the Tellius games and Fates (Conquest) to be high points of gameplay in Fire Emblem. Those games were deeply tactical and made me really think about each action I took. Three Houses, sadly, is not quite at their level. I think part of this can be fixed with a harder difficulty level, but part of it is rooted in the mechanics of Three Houses and some questionable balance decisions.
Presuming they make a concerted effort with DLC, there are a few directions I believe they can go in:
-Invasions of a hypothetical unified Fodlan (under your current house) by a coalition of other powers, such as Srengians, Almyrans, Dagda, Morfis and Albinea. This would involve loading up your previous completed save file and having to deal with a new conflict involving a coalition of predatory foreign powers.
-"Pre-LC" in which you take control of the ancestors or fathers and mothers of the characters in the game, including people such as King Lambert, Dimitri's father. This would involve playing a totally different story campaign.
-Alternative perspectives which introduce totally new casts of characters that occupy the same universe, perhaps from some of the foreign powers described above.
-Individual character stories which simply take existing characters and give them their own fleshed out mini-campaigns of sorts, possibly in conjunction with other characters (e.g. close characters like Ingrid, Sylvain and Felix could share a campaign). These could retain existing characters and introduce new ones.
-Simply adding on content to the base game, such as additional paralogues or even additional chapters pre and post-timeskip for each of the houses.