My party for this AP looks like this:
Human Sister in Arms Cavalier
Ratfolk Vexing Dodger Rogue
Changeling Invoker Witch
For the fourth spot, my player wants to run a Druid.
She's interested in focusing on Wildshape, because she loves Druids, but has always been more of a caster focused player in the past.
Any advice on something that would be thematically appropriate for this AP?
Well, if she's not attached to either an animal companion or a domain, the Feral Shifter archetype will let her lean a bit further into the shapeshifting aspect.
Natural Spell will let her cast spells while she's Wild Shaped, meaning she won't have to choose whether or not to be caster-focused or wild shape-focused. Druids have a pretty good spell list, too, especially in battlefield control.
Definitely download the Player's Guide, which should have a lot of good recommendations, especially for animal companions, if she goes that route.