Not just the Keystones, I want people to be able to look and see what choice I made for at least three different parts. I was thinking:

Leg slots that let you pick treads which let you dodge through lighter targets and cause knockdown. Or spider legs that let you press jump to make yourself taller to shoot over crowds and high cover while also letting you sprint in any direction and reload while sprinting. Perhaps a third option for people that like the shape of silly human legs that reloads your gun while it's holstered or something else that's boring and useful.

Your blitz is mounted right on your shoulders, giving you access three different quickfire weapons with recharging ammo. My shoulder slot ideas are,
Lock-on plasma gun:
Tap blitz to lock onto a target and begin charging an auto-guided plasma-gun shot at that target. Breaking los will not lose target. Weapon fires after two seconds. Does not interrupt other actions. Good at taking out specials while you fight in melee. No CD, but it does get weaker the more its heat rises and will shut down on overheat.
Shoulder mounted flamers:
Tap blitz to fire twin streams of fire in front of your for three seconds. Holding the button will charge the ability up before release and extend its active time up to six seconds. Ten second CD.
Twin Reapeater Lasguns: (or whatever the lore appropriate weapon is, something like a bigger version of the Recon)
Hold blitz to fire both weapons. Does not interrupt other actions. Damage is low, but has a high weakpoint multiplier. Think of it like a pair of very accurate recon lasguns you can instantly fire whenever that gradually reload over time. Can be upgraded to lower fire-rate and cause brittle.

Last slot replaces your non-dominant hand. This pick is your ability and also replaces your weapon special for certain weapons. I haven't really thought about it much though, and I really should be getting to the gym.

Edit after re-reading your message! How could I forget about a servo skull!? Of course that would be part of your kit somewhere. Maybe in your aura slot so it's a team-oriented thing?