With the coming changes and buffs to kobolds, are we gonna see anything similar for the cavern animal people tribes anytime soon, or is that for later? Right now their presence is almost negligible.
It's most probably for later. Animal people is a different set of issues than kobolds. The kobolds is a "civilized" major race with "settlements" that are being overhauled now, while animal peoples are split into "tribes" of underground animal peoples and "tribes" of above ground animal peoples, neither of which have any settlements or equipment beyond what's carried. At a guess, animal peoples would fit into the giant pile of law and custom thingies.
With the inclusion of different planes of existance, what kind of differences could planes have between worlds? For example, could we have a plane made entirely of gold filled with a single type of creature?
Also, how will these planes interact with the normal world?
That kind of plane sound quite possible. The first pass of inter planar contact will have one way portals that allow creatures from the other plane to get into the mundane world. The intention is to allow for player interaction to open/close (some) portals.
Later development will allow two way portals, but that's more than two major releases away (upcoming: artifacts, and thereafter myth&magic 1).