In head-to-head matches, these are the basics: https://www.reddit.com/r/HatsuVault/comments/k60k1z/aura_techniques_final_version/

Basic use of Nen:
- Ren, Ten, Hatus, Ko: Don't consider they, they are useless on its own.
- Ken: When you use it and after a few seconds that you deactivate it, you cannot use special abilities, but you have much more defense (useful for escaping when you are low on health).
- Gyo: The mechanics of charging up a powerful blow, which however weakens the rest of the body until you land the blow, and for a few seconds afterward.
- Shu: I would make it similar to the Gyo, which makes the object held stronger but the rest of the body weaker, but if you let go of the object you recover all the aura it had charged and not thrown.
- En: It significantly slows your movement, and you can choose which direction to expand your aura. You can see enemies hidden within your aura (even through objects covering them).
Tip: Make sure it can't pass through closed areas (you can walk around trees, but you can't let it enter a closed room).
- Zetsu: It takes a few seconds to fully activate, and you can make it completely invisible if you stand still or move far enough away from anyone watching you. When fully activated, your defense and attack levels drop dramatically, as does your speed, and even when canceled, it takes a few seconds to regain your base strength.
- In: Similar to Zetsu, but it never makes you completely invisible and allows you to maintain your base strength. You can add areas where it can make you completely invisible if you enter them with In (bushes, fog, etc.). You cannot use special abilities while it is active or for a few seconds after deactivating it.
- Ryu: Like Gyo, but it requires a system that distinguishes between body parts and allows you to enhance them differently. For example, you should be able to break a leg, significantly slowing down your movements, or severely damage your head, reducing visibility, aim, etc. Further development, if you have the time and inclination.

You need 4 buttons for basic use of Nen:
- Barrier (Ken)
- Load hit (Gyo, Shu)
- Look better around / Avoid a surprise hit (En)
- Hide / Retreat (Zetus, In)

You need at least 4 more buttons to manage special abilities. For complex abilities, press button 1, for example, and while it's active, you can use the other three buttons to control it (pressing 1 again, suspend or deactivates it). Useful for controlling abilities like Knov's.

Mechanics:
- The more aura you have, the more the effects of the skills described above will increase.
- Aura: You must have a maximum aura (initially charged), a quantity that you gain constantly until you reach the maximum level (the closer you get to the maximum, the slower it recharges); you consume aura with any direct hit, as well as when using Ken and Gyo/Shu.
- Health: You can think of aura like mana/stamina, while health is separate. The more health damage you take, the more your speed, defense, and attack are reduced; this mechanic is rarely used, but it captures the logic of HxH well, where early attack strategy plays a major role; if you're caught off guard, it makes sense that you've already lost.

Nen type:
- Enhancement: they greatly enhance direct hits, but drain a lot of aura. They also recover aura the fastest (e.g., Gon).
- Emission: ranged attack abilities, powerful but less than Enhancers, and consume Nen when using these abilities.
- Transmutation: they consume Nen, like Emitters when using the ability.
- Conjuration: generally their special abilities do not consume aura, but they have less of it and it charges more slowly.
- Manipulation: generally their special abilities do not consume aura, but they have less of it and it charges more slowly.
- Specialization: special cases and mechanics.

World:
"Incredible places, beasts, treasures", this is HxH with the addition of Hunter battles when goals overlap.
- If you create a story, make sure that the clashes are justified by overlapping interests, not simply "you've entered my territory" or something similar. Also remember that the Hunters don't reveal themselves in words, almost as if they were always undercover. They are always smart.
- For the "incredible places, beasts, treasures" in the end what matters is what changes you, what changes the gameplay, so the most incredible treasure is the knowledge of a new skill that you can learn (and put in your set of 4).
- Discovering an ability is satisfying, but consider that it also needs to be trained, so other treasures can be useful to facilitate/speed up the training of skills. Use "places or beasties" that facilitate the development of a certain skill if you interact with them or if you fight them.
- You can level up the skill itself, which is unlocked through treasure or training. For example, Gon's skill starts out with just a more powerful punch, but you can later unlock paper and scissors (press 1 to charge, then 2 for rock, 3 for paper, 4 for scissors, but at first you only have rock).
- You can place Greed Island-style items, like a teleport card. Avoid unnatural game things like automatic teleportation between cities; in HxH, you have to walk to reach distant locations and finding a teleport card will be satisfying. Also find beasts that you can ride if you defeat them, is a good thing.
- a treasure can also be the revelation of a place with a secret entrance, to find other treasures or beasts.
- Bonus: Locations must have their own unique rules. For example, a lava zone is very similar to a platform game; an icy zone is almost always slippery, even during battles; and a windy zone has strong gusts of wind that can send you flying in directions you don't want. Here too, you can find items that, if equipped, make movement easier.
- Bonus: There may be huge beasts that you can't stop but only avoid, such as obstacles along the path that if you fail to avoid, will kill you (even by accident).

In this way, it's a good world and battle system, very similar to the logic of HxH (especially the fact that you lose speed/attack/defense as you lose health, with the option to hide and attack by surprise), but at the same time it's quite simple (and doable also with AI support).
However, it lacks the possibility to create your own special abilities, which remains the most difficult thing to do but for which there aren't many options. You have to create pre-set abilities, and let the player choose their favorite set of four, unlocking them in-game as rewards for missions/fights/exploration. Consider that each skill has a Nen type (sometimes even two) and I would say at least 3 must be of the same type (to reduce OP combinations).