The idea of a human as a "host" has become a often explored trope in this day and age. Some common examples are: the Goa'uld, the Yeerks, Venom of spider-man fame, Starro's minions, the Bugs from Starship Trooper, Trill (to a degree), and many more. We've developed the book of Symbiosis to reflect this new trope.
The "Primal Host" base class. They utalized powerful symbiotes to devistate their foes. However... there is a price. You must allow your symboiote to "indulge" and use a fun new relationship mechanic to gain the maximum potential from your body's guest!
Symbiotic Item: New living wondrous items that bond themselves to you (I say "wondrous" but they are actually non-magical) Included are ways to design these items, a dozen new items, and some very creepy cloaks!