What resources do you have?
Out of the MM, both areana and doppelgangers are CR 3. Tacking on a few class levels (sorc for areana, rogue for doppelganger) would do them a spot of good. The areana would foreshadow the one encountered later in Here There Be Monsters, but I don't know if you want to double up like that.
Lycanthropy is a possibility - a were dire bat (although that'd get confusing with the varragoin flying around), a were constrictor, were caiman (use a Medium crocodile) or were jaguar would all fit the Tamochan theme.
If you have access to the Pathfinder series of modules, Pathfinder 2 has the faceless stalker, a nifty little CR 4 shapechanger. They get morale bonuses for fighting opponents who see them change shape, for instance, and can squeeze through tiny cracks.
Demiurge out.
