We've established some drawbacks and quirks associated with the quasi-mystical powers that characters can achieve...but what are those powers?
There are three general categories of power; psychic, fungal, and mutagenic, and six elemental forces, but these don't really mean anything.
Two of the first things I always consider when developing a setting are... What powers do I like from other games?....and...which of those powers make sense in this setting?
I've been reading Through the Breach, the Malifaux RPG, and it's got a great idea for magic. Characters might be necromancers, traditional sorcerers, or tinkerers. These different core types of magic provide different basic spell components, but each of the components may be mixed and matched with other to produce fully functioning mystical effects. It's basically the same principles as the magic system I love in Chill 2nd Edition, except that regular folks can be imbued with mystic forces to manifest supernatural abilities of a fixed type (true magic users can construct memorised spells, or mix and match ad hoc components on the fly). I could do the same sort of thing here, since we've got three general sources of power, and a range of elemental forces all of which need to combine into a coherent and logical system that a game mechanism can be applied to.
Through the three different sources of power, we've also got a bit of a vibe like the disciplines in Vampire the Maquerade which might be empowered by the expenditure of blood, or willpower, or by a simple die roll. (but in our case we're generally looking at sacrifice of genetic stability, risking fungal infection, or the possible metaphysical risk of opening one's mind to raw psychic potential.
I like the idea of set mystical powers, but powers that can be specifically constructed by the player to match their vision of the character. So the main thing I need to worry about here is the specific componentry of the basic effects, and then how those basic effects might be modified.
I'm also thinking of pulling the earlier idea of castes back into this part of the setting. But, instead of thinking about them as social phenomena, they now exist as guilds who specialise in teaching specific types of augmenting effects and mystic components.
Scavenger (Eyes) - Heightened Awareness (air/light), Jury Rigging (light/water), Stealth (darkness/water)
With minor access to - Innate Technology Sense, Empowered Weapons/Armour, Travelling
Scavengers favour psychic sources for their power, followed by fungal.
Scavengers gain easier access to modifiers giving longer duration or permanence to bonuses, with minor access to modifiers that reduce the difficulty, or modifiers that specifically target objects.
Warrior (Fist) - Heightened Speed/Strength/Physical-Power (fire/air), Natural Weapons (earth/fire), War Form (water/darkness)
With minor access to - Empowered Weapons/Armour, Mental Domination, Finding/Making Weakness
Warriors favour mutagenic sources for their power, followed by fungal.
Warriors gain easier access to modifiers that inflict more damage, with minor access to modifiers that increase the difficulty for people to resist the effects, or modifiers that require some kind of sacrifice.
Zealot (Heart) - Inspiration (light/air), Empowered Weapons/Armour (fire/earth), Spirit Affinity (light/water)
With minor access to - Heightened Awareness, Communication, Healing
Zealots favour psychic sources for their power, followed by mutagenic.
Zealots gain easier access to modifiers that increase the number of people affected, with minor access to modifiers that gain benefit from having allies nearby, or existing on/near sacred ground.
Scholar (Brain) - Innate Technology Sense (water/light), Intuition (air/light), Hiveguard Affinity (light/earth)
With minor access to - Heightened Awareness, Finding/Making Weakness, Fungal Affinity
Scholars favour psychic sources for their power, followed by fungal.
Scholars gain easier access to modifiers that reduce the difficulty of the effect, with minor access to modifiers providing more information about a subject, or modifiers that conceal the nature of the effect.
Diplomat (Tongue) - Mind Reading (light/water), Mental Domination (fire/darkness), Mystic Contracts (earth/water)
With minor access to - Inspiration, Persuasion, Communication
Diplomats favour psychic sources for their power, followed by fungal.
Diplomats gain easier access to modifiers that expand the area of effect, with minor access to modifiers that focus on a specific type of target, or modifiers that gain benefit from having allies nearby.
Merchant/Trader (Blood) - Persuasion (fire/darkness), Travelling (light/air), Communication (water/light)
With minor access to - Inspiration, Stealth, Intuition
Merchants favour fungal sources for their power, followed by psychic.
Merchants gain easier access to modifiers that focus on a specific type of target, with minor access to modifiers giving longer duration or permanence to bonuses, or modifiers providing more information about a subject.
Strategist (Bone) - Hive Mind (air/water), Finding/Making Weaknesses (fire/darkness), Shapeshifting (water/earth)
With minor access to - Intuition, Natural Weapons, Travelling
Strategists favour fungal sources for their power, followed by mutagenic.
Strategists gain easier access to modifiers that increase/decrease difficulties of associated actions, with minor access to modifiers that conceal the appearance of the effect, or modifiers that manipulate the time at which things occur.
...and maybe I'll add in a new caste/guild...
Nurturer (Breath) - Healing (air/light), Fertility (earth/water), Fungal Affinity (earth/light)
With minor access to - Inspiration, Mind Reading, Persuasion
Nurturers favour fungal sources for their power, followed by psychic.
Nurturers gain easier access to modifiers that make their effects resistant to countereffects, with minor access to modifiers giving longer duration or permanence to bonuses, or modifiers that require some kind of sacrifice.
...and then there are a few forbidden arts, because there are always forbidden arts.
Cursing (fire/darkness), Shellbrood Affinity (earth/darkness), Pyro/Hydro/Geokinesis (and whatever fits for air) (element/darkness), Necromancy (water/darkness), Animal communication/affinity (earth/light), Fleshsculpting (water/earth), Inquisition (fire/light), Illusions (air/water)...
I'm seeing each of the power types described having three core magic components...
Example 1: the "Travelling" power components might start with "Ease of Passage" which basically reduces the chances of running into detrimental situations by finding the best natural path between two locations, next would come "Haste" which speeds up anyone affected, and finally "Teleportation" which instantly transmits a person from one point of the Hive to another point they are aware of. These effects would typically be more appropriate as mutant abilities or psychic functions, the typical curse associated with this path of powers could be a gradually spreading tattoo map of every place the character has been.
Example 2: the "Stealth" power components might start with "Hiding" which basically directs the user to the most appropriate natural location for remaining hidden, next would come "Camoflague" which might actively colour the skin of anyone affected or even turn them translucent, and finally "Invisibility" which would render them unable to be seen at all (through some mental trick or stepping partially into the spirit world). These abilities could easily work as mutant abilities, fungal effects, or psychic functions, the typical curse associated with these powers might include people forgetting who you are and possibly even a risk of forgetting yourself.
The core thing here is that we're looking at interesting actions for the characters to do, and interesting ways to integrate these actions into the setting. Specific fungi will probably be linked to specific abilities, and likewise with specific mutagenic sources and specific psychic channeling techniques for certain powers.
I'm also seeing the potential for effects that combine core components to make interesting crossover effects, where most characters pick up a few minor fixed abilities as they ascend the ranks and uncover the mysteries of the Darkhive, but some open up new abilities or master the techniques for combining effects and modifiers on the fly. I'm seeing secretive mistresses researching new combinations to take their enemies by surprise, perhaps like the arcane scholars of Ars Magica, or even the reality bending magi of Mage: the Ascension. But it seems that the moment that for every element I pull back into alignment with my vision for the setting, two more exist as half complete ideas drifting in limbo.