0d6: You are completely untrained in this art. You can only attempt Small Magicks and they require a minimum of 2 successes to work properly. If you roll only 1 success, you will take a point of Magical Harm.
1d6: You have been trained in the basics of this art. You can now work Small Magicks more reliably and they only require 1 success to work properly. You can attempt High Sorceries, but they require 3 successes to work properly and 4 to work without incurring a point of Magical Harm.
2d6: You have received some advanced training in this art. High Sorceries only require 2 successes to work properly and if you roll at least 3 you don't take any Magical Harm.
3d6: You have mastered this art. Even High Sorceries only require 1 success to work properly (2 successes will enable you to cast one without taking Magical Harm).
If you roll at least one success on a spell, but not enough to actually get it to work properly, the spell goes awry and has a related but less focused and poorly controlled effect. A common one is for the energy to simply be wasted by spreading it too thin.
If you score even a single success when attempting a High Sorcery, you take a point of Magical Harm unless you roll more successes than the necessary minimum.
Making a long-term version of a spell increases the minimum number of successes required.
Okay, another attempt at a really minimal skill-list, inspired by the set from Arkham Horror (Speed, Stealth, Fighting, Willpower, Lore, Luck):
Acrobatics (jumping, climbing, swinging, dodging, etc.), Fighting (melee and ranged), Stealth (includes most thievery), Lore, Luck, Magic.
So... split Fighting into ranged and melee so that we support good archers as well as tough warriors. Stealth and Thievery. At least two kinds of Magic. History and Knowledge?
Hm. Basically, I think we should split it between Ancient Lore and knowledge of modern day conditions like what various areas are like. Fallows Lore, perhaps?
World Lore?
Ancient Lore?
Okay...
That's 4 Athletics skills, 3 Brawn, 4 Cleverness, 2 Determination, 2-7 Magic (depending on how you divide up the Arts). Determination has the fewest skills, but honestly it helps them all by enabling you to take more Harm. One more Brawn skill? Okay, Endurance makes 3. What about healing skill? Or Herbalism? Or survival? Lump 'em all under Wilderness Lore, I guess. Except maybe healing. Hm.
Maybe a separate Endurance skill? But it seems like that kind of overlaps with Resolve... do I want separate "physical toughness" vs "mental toughness" skills?
Okay, this gives us... 2 Determination skills, 3 Brawn, 4 Athletics, 5 Cleverness and up to 7 Magic (but only 1 of which is actually useful if you have a Magic rating of 0d6). 5 spell skills would give us a nice numbering...
Now I'm at 2 Det, 3 Brawn, 4 Ath, 6 Clev, 7 Mag.
Tug, Spark, Whisper, Scry and Compel. I'd have to lump Shaping under Tug, though... although that does kind of fit with "tugging things into new forms". I dunno... it might be worth separating them so that you can have a mage who has TK but can't shape or vice versa. Up to 7 Magic skills isn't bad.
Okay, a quick setting thingie... can a mouse with a Magic rating of 0d6 even attempt an actual spell? If I allow 0d6 ratings for the other stats... hm. As long as I enforce some sort of Harm coming to you when you fail a spell-casting check, it could be workable. There's a huge difference between a common rat who has to spend lots of time to get even 1 success and a white mouse mage who can reliably just toss a spell off.
0d6 = utterly incompetent
1d6 = poor
2d6 = typical
3d6 = good
4d6 = great
5d6+ = fantastic
Okay, Unopposed Aid rolls are called... what? Stunts? Synergies? The idea is that you combine two skill rolls... successes on the first add extra dice to the second. If you fail to gain any successes, does your secondary roll automatically fail? Or just receive no benefit?
Opposed Aid rolls are Manuevers. You Manuever for Advantage. Your roll is contested by theirs, each success you have above them grants you +2d6 to your follow-up roll.
Each one is one or two skills that basically get blessed under certain circumstances. The more restrictive the circumstances, the more skills you get. Ideally, do two themed skills with a matching restriction.
Lone Wolf: when you travel alone, cut off from any companions or ready reinforcements, your Stealth and Wilderness rolls succeed on 4+.
Snake Charmer: your Animal Handling rolls succeed on 4+ against dangerous (but unintelligent) predators. Your Charm rolls succeed on 4+ against intelligent predators.
Assassin: your Stealth rolls succeed on 4+ when using them to aid attack rolls.
Lady's Mouse or Femme Fatale: your Charm rolls succeed on 4+ against people who might find your character romantically attractive.
Cornered Rat: when you've unwillingly ended up in a spot where retreat is impossible or suicidal, your Fighting rolls succeed on 4+.
Marksman: your Perception rolls succeed on 4+ when using them to aid Shooting rolls.
Bladedancer: if you have plenty of space to whirl about, you can use Athletics+Perform to aid rolls with swords or knives. Your rolls succeed on 4+ when doing so.
Mighty Mouse: when you make a Brawn check to lift or break something, that check will succeed on 4+. Squeezing someone you have grappled counts as well, but not other Brawn checks.
Slippery as an Eel: Stealth and Escape rolls made against foes who aren't looking at you, or against inanimate objects, will succeed on 4+.
Silent as a Mouse: when you are find a hiding spot and remain there practically motionless, your Stealth rolls succeed on 4+.
Dirty Fighter: when you use Trickery to aid a Fighting check, your Trickery roll succeeds on 4+.
Politically-Connected: when you roll Status to aid other rolls by calling in favors, your Status roll succeeds on 4+.
Danger Sense: when you use Intuition to get a feel for how dangerous a situation is, your roll succeeds on 4+.
Specialist Mage: choose a single Magic skill to be your specialty. When you use that spell skill in a calm, non-combat situation, your rolls succeed on 4+.
One-trick Mage: choose a specific effect of a single Magic skill to be your specialty (throwing a fireball with Spark, for example). When you cast that particular spell, whether under stress or not, your rolls succeed on 4+.
Inner Power: when you cast a spell, you can choose to take 1 point of Magical Harm before you roll. If you do, the dice that you roll for that casting succeed on 4+.
Shaman: when you use spells on spirits, your rolls succeed on 4+.
A replacement Knack can be purchased during character advancement ONLY if you didn't use it at all that session. You can voluntarily forgo using a Knack if you intend to get rid of it at the end of the session.
Stats: Athletics, Brawn, Cleverness, Determination, Magic
Skills: Acrobatics, Animal Handling, Burrowing, Charm, Escape, Fighting, Intuition, Leadership, Lore, Perception, Perform, Resolve, Shooting, Stealth, Streetwise, Trickery, Wilderness
Magic Skills: Spark, Tug, Whisper, Scry, Compel, Shaping
So, Cassandra would have decent Spark, Tug, Compel and Shaping.
Spark: create light, fire and static charges.
Tug: move things telekinetically.
Whisper: communicate mentally (includes speaking with demonic gods).
Scry: expand your senses to pick up things normally invisible to you.
Compel: project a mental compulsion onto others. Can be subtle (don't notice me, manipulate emotions) or overt (hold at bay, paralyze).
Shaping: reshape an object by touch.
The eternal tradeoff... offering cool powers vs keeping the system simple.
If spells are the only way to do anything "nifty" then everyone will want spells, right? Or will they?
Yeah, I tend to think so. D&D and such get around this by giving bonuses/abilities/feats to people who choose particular roles.
Only CoC strikes me as a game where differentiation is mostly not through special powers, but it has a very un-cinematic system where you can't have all of the skills.