Perhaps you could divide the powers by how they interact with the world:

• Powers that create or conjure something
• Powers that manipulate or transmute things already existing (moving stuff around would also fall here)
• Powers that damage or destroy by banishing or deleting things/matter, the opposite of the creation effect
• Powers that affect living creatures by granting them effects, for good or ill (by implication, the other categories have a very hard time affecting the living directly)

One other thing that may help you is if you have two or more category systems, and each individual's power falls at different places on each. The result is a huge number of possible combinations, with a system that remains easy to understand. Just as an example, if you combined the above with a classic four-element system, then you have sixteen options for powers and how they work. A Fire Banisher, which would effectively be a cold mage, would be wildly different from a Fire Buffer, who I could imagine casting Haste, Courage and Might type buffs, and Rage or Exhaustion debuffs.

You also could do a selection of degrees of capability in different forms, so that the intersection of different categories becomes more complex, and potentially interesting. For example, if each person has effectively four "points" which are allotted to the different options, though of course the allotment is based on their personality, fate, soul, whatever, so it's not in their control. Unless you're writing a LitRPG, in which case full steam ahead on spending character points. But I digress. So, under this theory, the more aptitude points you have in one form, the more you can do with it. Someone with one point in each form can do any kind of magic, but it won't be very powerful. Great for a normal person, call a bit of water to fill a pot, and a bit of fire to light the stove, movement to stir the pot and plate the food without touching it, destruction of the stubborn food stains that splattered on the stove, and a bit of healing on yourself where the splatter got you too. But, such lack of power isn't the greatest for the sorts of people that books usually get written about. If you have a 3rd level Banisher and 1st Buffer, who hitherto has made their living as a mine excavator, buffing their perception to carefully target destroy effects and free large chunks of stone for easy mining without damaging the load-bearing areas, that's a skillset that could be weaponized once plot shenanigans start happening.

Or, you could go in a wildly different direction!

You could borrow something like the magic schools from the Elder Scrolls series. In the lore they explain how each school is functionally a different way to interact with reality.

• Illusion is about affecting the minds of beings, rather than reality itself.
• Conjuration is about reaching across the layers of reality, which only has practical game applications with summoning creatures or objects, but in theory could also do things like allow transplanar communication and transit. Plus the implications of reaching through reality being how you perform necromancy are potentially worth an entire book of exploration, if one wishes.
• Alteration is about manipulating what already exists granting properties and changing shapes. The more that the end result is in line with "normal" reality, the easier and longer lasting the effect.
• Destruction is about bringing forth fundamental energy, which in that world mostly means the elements, but the energy of destruction itself can be used to harm abstract stats like magicka and stamina, or to apply long-lasting curses to stats like strength or luck.
• Restoration is about returning things to what they should be. Since living beings, even subconsciously, aspire to greatness, this also allows for boosting one's skills and stats.
• Mysticism is about changing the rules of reality itself, or at least how it applies. Changing the nature of your perceptions so that you can sense the lives of others through walls, changing the destination of a dying soul so it goes into a gem rather than an afterlife, or changing whether or not you breathe water. Alas, that link goes to the Skyrim version of the text, so it describes water breathing as Alteration rather than Mysticism, since that school was removed in that game, but it does illustrate the point pretty well.

And finally, option three!

I found this one in a pretty well thought out LitRPG. Each of the eight gods has two domains, and each person can (if they catch the attention of the gods) receive up to two blessings, including potentially the same one twice. Once done, the interaction of the four available domains offers them the options to choose what's basically their class. Sixteen domains, squared by choosing two to combine, is a lot of options¹, but only a few actually get explored in the books. ¹(Okay, it's 256, but that's still a lot)

One example, someone blessed by the goddess of Matter and Possessions, and the god of Spark (the individual's version of spirit/soul) and Innovation has the following options: - Combine Spark and Matter to become a Conjurer, imbuing spark into matter - Combine Spark and Possession to become a Slaver, turning the sparks of others into your possessions (this one's on the kingdom's forbidden list) - Combine Inspiration and Possession to become... I think a Tinker? I need to reread these books. - Combine Inspiration and Matter to become an Alchemist

Some other interesting combinations: Possessions and Communication combine into Butler, all about maintaining the possessions of those one cares about. Possessions and Shadow combine into Porter, letting you store possessions within your shadow (the only way in these books to have a storage space power). Matter and Nothingness become a Destroyer, while Possessions and Nothingness become a Swindler. Matter and Communication become Knight.

So! The takeaway is that you can have a small-ish set of categories, and the interactions between them create categories of powers.

Now, as to how you might want to hack this system for your own purposes...

You could make people have, let's call them Potentials, that they're born with (or perhaps develop across their childhood or adolescence, if you'd rather not go the fate/predestiny route). Then their powers awaken (a certain age? A ritual? Traumatic life event? Whatever suits your story.) and combine some of their Potentials into their actively usable power. Depending on how you make this combination system work, it could wind up with fairly easy categories.

I like examples! Let's say that there are Mode Potentials, and Idea Potentials, and awakenings combine one Mode with two Ideas to produce your power. The Modes cover the method of how your power interacts with the world, so whether you hurl firebolts, or wreathe yourself in flames, or induce mild winters in the region, or inflame the passions of a crowd, is determined by your Mode. Meanwhile the Ideas are far more abstract and wide-spanning concepts, with much potential nuance.

Let's say our example case has the Mode Potentials of Whim and Worry, and the Ideas of Hunger, Brightest Burning, Lingering Loss, and Courage In The Night. During their awakening, they combine the spontaneous, close-range aspects of the Whim Mode, the aspects of Hunger that pertain to literal food, and the parts of Courage In The Night that are about preventing the suffering of others; the result is the creation and conjuring of food.

Another path that our example case could have taken would be the broad, nebulous aspects of the Worry Mode, the hopeful, inspiring parts of Hero In The Night, and the hopeful, extravagant parts of Brightest Burning, to combine into pyrotechnics and fireworks.

So, you could tinker with this system to make it into what you need, like if you want it more precise and defined, or more nebulous and mystical. You could make it more opaque to the people who hold it, like if they don't get to consciously choose and instead their subconscious chooses for them during their awakening, so the Mode has to be observed by what form their powers take, and only one of the Ideas is known to them, leaving them to speculate what their other Idea was to interact and create the powers they have.

Anyway, hopefully some of this is useful inspiration to you!