They have yet to introduce a single Tier-4 B-skill, and I doubt they ever will. Having a signature B-skill means you can improve your score cap, and is a selling point for legendary units. They wouldn’t make that available as a general, inheritable skill; it would devalue the legendary units.
I could easily see Close Defense 4 on either of Edelgard or Dimitri. +8 and negating all visible buffs on melee foes isn’t really that substantial, truth be told. A lot of the game focuses on combat buffs now, so that’s not only possible, but wouldn’t even be that substantial.
Defiant 4 skills would be interesting, but I think those would also be hard to justify as an A-slot skill. Unlikely, I think, but definitely needed. It’s just tough to figure out something that isn’t just…Brazen but worse.
Breath 2 skills would be something, especially if they combined two stats. But Ike isn’t here so I dunno about these Brave units being able to breathe.
Breath of Life 4 is what I want, though I could see it becoming a self-heal as well. After combat, heal up 7 to self and nearby allies. That’d be amazing if it could work with healers, get my Absorb Tank Lachesis running. If Lysithea is a staff unit, I could see that happening.
Given that L!Edelgard’s B-skill was Raging Storm, I’m guessing their unique skills are set to to be for the legendary alts, so maybe their gambits from their default battalions will be their unique special skills as Brave units? So Edelgard has Raging Flames, Dimitri has Wave Attack, and Claude has Ashes and Dust, all as AoE attacks that bypass damage reduction, because they’re eventually going to introduce something that bypasses damage reduction so may as well get it out of the way.