It’s very likely at this point that Saltz’ next career will be a Sigmarite warrior priest. Sienna’s though seems a little up in the air. Ultimately she isn’t going to change her wind of magic for lore reasons and because it would invalidate every staff she has so far.
One suggestion I remember seeing is that she could become a Knight of the Blazing Sun. This makes sense as they allow untraditional forms of combat such as archery into their ranks and use fire imbued weapons. Sienna clearly shows devotion to Myrmidia as well. Question is how do you make this different enough from Unchained which is already a melee focused tanky build? What would her career skill be?