Saving the most complicated for last:
The bionic: An electricity harvester-type thing. Activating it would disable the dielectric capacitance system bionic if it was active before. It would appear on the active bionics list, but be toggle-able with no power drain. How it works is this: Taking electrical damage would give you some bionic power back. It wouldn’t function if you’re somehow grounded (which is why it forbids the use of the dielectric capacitance) – Also wouldn’t work with the Faraday shark suit or grounding artifacts. You would HAVE to take damage for it to work, so players couldn’t just sit and let a shocker zombie charge them to full.
The gameplay streamline: Being able to “attach” a style to a weapon from the inventory menu, so that when that weapon is wielded, the character would automatically change to the specified style. Say for instance the player uses karate. They would be able to go onto their combat knife’s inventory menu and assign Silat to it. Draw the knife, the player is automatically changed to Silat. Sheathe or drop the knife, changed back to karate. It would simplify without dumbing down, and eliminate a teensy bit of micromanagement.
Requiring the most explanation is the mutation tree: I don’t know how we collectively feel about the scribbled-in-the-margin-of-a-personal-notebook kind of mutation trees, like elf-a, but it would be like that: Vampire. Combining elements of troglobite (for light sensitivity/damage and night vision) and cat (for claws and fangs), plus a taste of a couple others, but mostly those two.
Positive mutations associated with this tree would include the aforementioned night vision, claws, fangs, maybe strong/dextrous buffs, less sleep, resistance to infections, fleet-footed maybe leading to road runner, and/or light step. The player would also gain the ability to gain nutrition from blood (only from living things, never zombies). A post-threshold mutation enhancement on Fangs would be similar to the Water Extraction Unit CBM – Allow them to draw blood from a corpse and store it in a container for later use. There might also be a post-threshold enhancement on claws, making them stronger and unique to the tree. Possibly another post-threshold mutation would be similar to the dummied-out Inconspicuous. When standing still, the player character would be harder to spot.
Negative mutations associated this could include any of these: The light sensitivity leading to solar sensitivity, pale skin leading to albino, animal discord and/or strong scent, all of the various “normal” food intolerances: meat, lactose, junk, wheat, etc. Fast metabolism. Flimsy. Ugly leading to deformed (think Orlok from 1922’s Nosferatu). Wing stubs leading to bat wings.
This is mostly a joke, but there could even be an opposite of Spiritual, where carrying or being near “holy” objects or scripture causes minor morale penalties.
The tree would culminate with Hematophagy, which would disallow all other food sources but increase the nutrition/quench value of blood.
In short, the tree would make the player well-suited to night raids and stealth-oriented melee combat, but poor in the daytime and a bit on the fragile side.
EDIT: Forgot a tree-specific negative mutation. Bleeding would cause the player to lose hunger/thirst quickly.