It’s a flip of a coin really. Most of the time a battle box one and done for both sides and afterwards it just your army rulebook ( Hexfire, Shadow Throne, Aether War, and Carrion Empire). Sometime it leads to one side getting stuff later on and other side is generally just a book (Piety and Pain, Pain and Pleasure, And maybe Saga of the Beast and Blight Wars). There was few time that the boxes where tied to a new release with the battle box having actual new miniatures in it for one side (Feast of Bones and Looncurse (may have been issues)). Then there those rare time where the battle box contain new miniatures and it lead to new miniatures for both armies later down the line ( has only been in 40K, with Shadowspear and then now with Eldritch Omens, though I guess technically that weird Wraith and Rapture box counts too I guess)
but hope lead to disappointment so with no buzz about new models for FS or Deepkin it hard to hope for more atm.