A quick brainstorming led to some "catchy" skill names for the characters, such as "Plundering Aura" or "Scurvynator" pirate skills, "Additional arms" robot special upgrade, "Thousand knives raging lightning storm rush attack technique" ninja technique, and "Spare Parts", "Climactic break-in" and "Vomit Maggots" Zombie attacks. And some more that I won't be mentioning (I have 18 skills for each of RZNP, total 72 skills).
My quick brainstorming also led me to creating four player abilities to replace the traditional "Strength, Dexterity, Intelligence" ones:
Brutality - The ability to deal huge amounts of damage to anything in your path, blindly bashing anything however hostile. You also become (obviously) less stealthy. Whenever you strike someone, your brutality increases (and returns to its initial value when you stop fighting), so a good strategy for Brutality-heavy characters is to always be fighting. Brutality is a main skill for Pirates.
Equanimity - The ability to keep one's concentration during a fight. Equanimity-heavy characters are able to strike with a very high accuracy and inflict high damage. Equanimity decreases as one keeps striking, so a good strategy here is to cease fighting and flee, only to return when one is concentrated again. Equanimity is a main skill for Ninjas.
Rapidity - The ability to move quickly, whether to run, to dodge hits, or to strike with a weapon. Rapidity helps for surprise attacks and for characters that do not want to get hit often. Rapidity is a main skill for Ninjas. Rapidity decreases when tiring actions are performed.
Stability - The ability to withstand hits, and not take too much damage from them. Stability is also required to be able to strike or move under straining situations (such as being hit by multiple opponents). Stability decreases as the character is hit.
Robots have natural Rapidity and Brutality, while Zombies have natural Equanimity and Stability.
Robot characters get bonuses from the many upgrades they can use on themselves (additional arms, increased ability values, and even the possibility to use hundreds of weapons at the same time). Zombie characters get much more resilient as they level up, and will be able to call on hordes of undead for help at the highest levels.
In the end, there are four possible character strategies here:
Pirate Zombie: A nearly indestructible, stupid warrior that, given enough time, will be able to kill just about anyone.
Pirate Robot: A killing machine able to kill anyone with one hit (of its six swords).
Ninja Zombie: A stealthy killer, with a still mind (or whatever zombies have for a mind) and able to relentlessly strike at maximum efficiency.
Ninja Robot: The fastest thing around, this martial artist runs, jumps, turns invisible, slices and dices.
Back to work...