I was thinking of the following:

Vessel Slot:
Allows you to install vessal upgrades that may boost the body by in regeneration.

Nervous System: Could be used to boost essence.

Eye Slots
Something that gives you a radar.

Brain: Something that allows you to control a monster

Legs: Something that allows you to do a ground pound

Lower Organs: On Death, something that makes you explode.

For that last suggestion, can the explosive power have various levels? You know, for those times when you want to go out like the Predator. Lol