God if Fortune's Weave is giving us different factions I hope to GOD you can't do cross recruitment.
Hot(?) take the recruitment system in Three Houses sucked and whatever good points it had (like how recruiting Caspar on Blue Lions gets you an extra scene or insights) are outweighed by the problems:
Like with the reclass system just letting everyone have access to every class, it doesn't encourage variety, it encourages using the same good thing for every route. Every one of your female magic users is probably going to end up a Gremory, everyone recruits Lysithea to Dark Spikes T the Death Knight.
This is especially bad because a lot of maps are reused across routes. If you were limited to just the initial class, then you'd actually have to use different strategies and think about what you're doing. A map full of armored knights is going to be a bigger problem for the Blue Lions than the magic heavy Black Eagles. Maps with a lot of fliers will be different for the Golden Deer than anyone else. Coughing Baby vs Hydrogen Bomb ass example here, but V'ehxness could very easily become a Death Knight style meme with how many times you fight her, because once you've got a handle on the mechanics you can do a stupid amount of damage. BUT that fight can be legitimately stressful even 50 endings in just by changing the units you have access to. What happens when your reliable damage dealers are replaced by a squishy healer, and tank who can't attack without literally becoming immobilized for a turn (also the fewer characters you have the less AP you have so you have to be more careful). Hundred Line has literally the same map for every battle and yet it feels different by limiting your access
Another thing Hundred Line does is actually build intrigue for other routes! You will find information out on one route and then start wondering what's going on in that route you passed three choices back. Characters will mention things you have no context for until 10 hours later and you're like "OH FUCKING SHIT SO THAT'S WHY" You're not going to play Crimson Flower and then play Golden Deer because you want to know Lysithea's deal, because you already know it because you recruited Lysithea. That means the only possible intrigue can be the lords themselves and their retainers, which is kind of limiting, especially for Edelgard or Dimitri, who don't come off well if you play the other route first! And it doesn't necessarily discourage playing multiple routes, it means people playing multiple routes are bored out of their minds because they're only getting a couple of bits of new information.
It fucking sucks for the characterization of just about every character. We can argue until the cows come home about why certain characters could be convinced to join one side or another, but the fact of the matter is that textually, Ingrid will abandon her homeland and then turn around and evade not because of anything to do with Edelgard, but because she had a (presumed dead) teacher who was really good with Pegasi for a few months. And that means we miss out on fun characterization for the lords, because we don't get to see how they actually inspire people to join their cause by having to persuade them, and we miss out on fun characterization for everyone else! What a character will never do is just as important as what they will do. The fact that Hilda will join in Azure Moon and Silver Snow but not CF says way more about her than the fact that you can recruit Raphael on all three routes. It just makes it seem like most of the characters don't actually have closely held beliefs?
I could live with it characterization-wise if we could just get something closer to Three Hopes, where not only are more units route locked, there's actually an opportunity cost to recruiting units because it means giving up on some advantages in battle so it requires some strategy instead of running around getting everyone, and it requires actually convincing someone after you beat them. When you recruit Shamir on Scarlet Blaze iirc Hubert says something like "if you join us, we'll spare Catherine" which is interesting for everyone involved.
(Also blah blah blah if Eshmel is the Sothis and doesn't actually exist on the physical map for most battles so recruitment is achieved via being magic that'd be less characterization breaking but still kind of boring)
However my point about the map thing remains. Contrary to popular belief I think you can have interesting storytelling AND good gameplay, and in fact those can go together, and one way to do that would be to put restrictions on the player for the love of GOD
(My absolute ideal is less different routes and more Shadows of Valentia/Radiant Dawn where you swap between all four protagonists' perspective of the same narrative, and then everyone comes together at the end, so at the least there's some opportunity cost between recruiting people to one army over the other)