So I'm planning to add a mechanic that allows players to create their own special moves...

So I'm planning to add a mechanic that allows players to create their own special moves, similarly to that of Hatsu from Hunter x Hunter.

I came up with two rules as far as what you can and cannot produce with this mechanic, the limitations being.
>It must be thematic to your character’s concept.
>It cannot involve concepts which the character has no knowledge of.
What I want to know is, how much can you break under these conditions and what rules are required to make it more "balanced?"

Also, before anyone asks, the setting does not have nukes or anything that could function similarly to that of a nuke. Just getting the obvious shit outta the way early

My thematic concept is [magic/imagination/energy/SCIENCE/living matter/mind/being fucking awesome] and my character is an expert who knows all there is to know in the field.

In order to have something like this work, the special moves need to be mechanically designed beforehand, then the players flavor it with their theme. Or you give the players specific concepts/ themes, then turn the game into how much you can bullshit with that theme.

First, make a list of every possible thing about a power that could be measured and compared, like range, area, mass, duration, complexity, specificity, etc. You can't begin to balance a custom power system otherwise.

Have you looked at Anima and Valor? Because both are anime-inspired systems that do this.

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So a generally good doctor could theoretically make special moves that can shut down his "patients" bodies?
What if someone designed their character to be a magician? Could they just make their opponents disappear?

I'd presume the more permanent or 'powerful' the effect the user perceives the more energy it would cost, so like a one shot would drain a lot while say a dodge using cards or something would cost very little.

Assuming you're going to make it scale with character level to keep it relevant I'd follow this rule:

>Damage should be equal to a spell of the casters level if it is a spell like ability or some sort of blast
>In case of a weapon attack, technique or martial trick. Make it damage as much as crit with a normal weapon would
>If its something they can only do once per day, you can raise the power levels, if its something they can do often, keep it so its interesting to use but not to the point where it will make everything else look useless
>Extra effects should be kept in the realm of reality, extra dimensional bullshittery is a a head ache and a half.
>Never allow players to have something with a Death effect

I love the idea of giving players something of their own making and I usually encourage mine to try and create things for their character, but usually both you and the player must be on the same page in terms of what they expect out of it. You gotta be honest and keep it balanced, the player should be rewarded for doing the thing he wants more than the effect it produces. Never enable someone to make something just out of convenience.

System?

One word: semantics. Once you're focusing on the "thematic" or "metaphorical" definition of a power rather than how that power would logically work, you can do pretty much anything.

For example, let's say you've got a guy whose theme is fire magic. That's pretty versatile. Fireballs, fire punches, fire swords, fire walls, fire breath, no problem there.
Creating light? Fire can do that.
Firey explosions? Sure.
What about fire constructs that act like solid matter and don't burn things unless the user chooses to, like a flame sword that can block steel blades or a flaming surfboard that lets him fly? That's not how fire actually works but you could say it fits the theme of his power.
Can he control magma? It's made of earth, not fire, but it's certainly firey.
What about making fire elementals who have their own consciousness and can follow orders independently? Creating a conscious being is basically an entirely new power in addition to fire control, but there's still fire involved.
Can he transform his own body into fire and then change back to normal? Fire doesn't reassemble flesh after it burns it, but he's the fire guy.
Could he use fire to "ignite" someone's anger? Can he replenish people's energy or strength? Can he heal people by infusing them with the spark of life? Can he dispel illusions by burning away lies and deception? There's a whole lot of stuff you can fit into the fire "theme" if you're willing to do so.

Basically, the limitations you described aren't very limiting at all. If you want to balance things while still allowing of pretty much any sort of power, you're looking in the direction of superhero RPGs Mutants & Masterminds. The genre is inherently about the extraordinary, so their systems tend to cover just about everything by focusing on the effects of the power and charging points based on what you can do with it, such as a ranged attack, or a binding effect, or flight. The themes of a person's power are a matter of fluff.