Last thing I wanted to post in way of prep is I'm thinking of emphasizing the "primal changes" the Beastlands induces in visitors. If you're unfamiliar, in the 2e Planes of Conflict box it described how visitors adopted animal traits and their "wild side" could come out (fast for chaotics, slow for lawfuls); there wasn't any rules effect of this, it was just a feature of the plane.
I'd like something that is also a bit psychological as one adopts the mindset of an indigenous person, Druid, lumberjack, etc. For inspiration on how to handle this rules-wise I'm looking to mechanics over here:
The Gray Languor Stormin' Da Castle, which uses the idea of horizon (timer) to deter excessive resting developed over here:
Don’t Rest Here : Menace and the Horizon
My version for the Beastlands is the "Urge." Essentially there are 3 stages of "Urge" the party can go thru (with some kind of differentiation for individuals), each progressively moving the PCs to casting aside the trappings of civilization and "going native". Certain things the PCs do can increase/decrease the Urge, though extended rests definitely will increase it. And it's not meant to be a game-ender if the PCs give into the "Urge", more of a game-changer.
I'm also bearing in mind that the Beastlands is essentially an Upper Plane, savage and wild at times, but good aligned. So the "Urge" won't turn PCs against each other or cause them to commit evil acts.
If anyone has ideas I'd love feedback!