I think a series portraying the division and downfall of Arnor, or the kinstrife in Gondor would be an interesting concept for the spin-off series. And it can tie in with most likely the end of RoP if the show portrays the establishment of those two kingdoms.

Edit: I just realized because Amazon bought the rights to the appendices of LOTR they can pretty much make a Fourth Age show around the time of Aragorn’s time as king or even his son Eldarion.