Newest Famitsu article is out, detailing more 3H info.
The game is heavily inspired by FE4. A previous war broke out and was quashed by Ten Heroes, and their descendants possess Large and Small crests as proof of their lineage and give them a specific aptitude in combat. So that's essentially Jugdral's backstory, right down to Major and Minor Blood.
The Adrestrian Empire used to include Faerghus and Leicester as its territory but those two countries broke off and act independent. Though, they did try to assert themselves as the dominant country on Fodlan, and the Church of Seiros acting as mediator is the only reason they haven't torn each other apart yet. So there's a bit of Tellius sprinkled in as well.
Gameplay wise, there is a calendar system like in Echoes, but it's much more involved to the point of it being almost being a life simulator in some ways. Every chapter includes one month, with the major fight of the chapter taking place at the end of the month. Until then, you have the remaining 29 days to plan side activities to improve your students, from teaching, to bonding, to sortieing.
Fists have an innate Brave effect, but to compensate, they can only be used by infantry and armored units. No fisticuffs on horseback. Skill system is like Fates, where everyone has a personal skill, and the skill pool is limitless, but a unit can have only 5 skills active at a time.