Good magic follows TANSTAAFL There Aint No Such Thing As A Free Lunch.
Magic must have a cost. Magic must have side effects. Magic must have limits. This, to me, is one of the disappointments of the Harry Potter books. Stuff just happens with latin derivative words.
Cost can be in multiple ways.
Mana -- a resource to be used. Larry Niven's fantasies have Mana as a resource geographically distributed. It can be used up, and only very slowly recovers. Other worlds ascribe mana to living beings. Bacteria have very small quantities. Humans have much. You can power magic with spells that kill. Power small spells with blood.
Personal life. Magic uses up your life force. variation of above.
Conservation laws. Magic is still subject to conservation of energy and momentum.
This opens up the possibilities of different skills of magicians:
A first level mage conserves using his body. E.g. If he throws a force bolt at an enemy he gets the recoil. If he throws a fire ball he gets cold. If he teleports down in elevation he heats up. (The energy had to go somewhere)
A second level mage can do two spells at once, one having the direct effect, one handing the side effect. E.g. That force bolt is anchored to the castle foundation.
As an example of conservation magic, consider the magic carpet. As long as it flies level, it takes almost no energy. This means that cities at the same elevation above sea level have really cheap trade.
There may be cities in steep terrain that act like 'locks' Teams of mages that can handle heavy energy that raise or lower your skyship.
This last one is an example of how the use of magic shapes the culture. This sort of detail is the difference between a kid's fantasy and world building.
A more ordinary mage on the skyship can exert a traction force between the ship and a distant point to pull the skyship along. A tier 1 mage, can pull himself along, bringing his carpet with him.
Mass is conserved? Does 150 pound man become a 150 pound wolf? If he doesn't where does the mass go? Can you have a were-ant? An amusing variant of this: Is fat conserved? Does a fat man morph into a chubby unfit wolf?
Full shapeshifting -- Odo on Deep Space Nine -- is too big a power. Such a person is unkillable, can go almost anywhere. Superman was cursed with vulnerability to a bunch of kinds of krptonite to make him vulnerable. Otherwise he risked nothing.
Another cost can be pain. Shapeshifting is painful. In essence all your muscles are torn, all your bones broken. Perhaps shape shifting has a recovery period. What is the effect if the shapeshifter has to take an hour to get used to his new shape, and is weak as a kitten during that hour. Shape shifting takes enormous energy. Suppose that shape shifting cost you 10 pounds of stored fat. Instant crash diet. If you don't have fat, it uses muscle mass. 10 pounds of fat generates a lot of heat. This may be the limiting factor in how fast you can change. Jump into ice water and change faster. Changing shape always leaves you ravenous. Changing to a wolf means you really want to eat the first toddler you come across. Perhaps why were-wolves have a bad rep.
In several of the Discworld books, a witch can ride along with an animal. See the world through hawks eyes. The cost: A human mind doesn't fit well into a bird brain. If you stay too long, you forget how to come back. If you only skim the bird mind, you don't have full control of the bird.
Scale of magic
Different practitioners may operate at different scales: Consider a telekinetic mage who can excavate a basement for you, versus one who uses his skill to pick locks. Or one who can get a chemical reaction to happen the right way.
Or a time mage who can see 27 seconds into the future routinely, but events further than that aren't possible for him. He knows the outcome of a the dice, but not often time enough to take advantage of it.
A thermal mage who can keep his coffee warm, but meets his limit boiling an egg.
A mind reader who has to touch his subject (My mind to your mind....)
A teenager who unwillingly broadcasts his/her emotional storms affecting everyone within 100 meters.
Zenna Henderson has a delightful set of short novellas/long short stories about the People. Aliens, like us, but with Powers and Persuasions. All can fly, all can read minds. But others aren't as evenly distributed.
One girl discovers she's a sorter: An extreme telempath that can feel other people's troubles and help them out -- but at great mental anguish to herself. The key is learning to channel the pain away and reduce it.
Various kinds of "lifts" (where you are moving something not you.) The most common requires sunlight. Moonlight is more powerful (?) Midnight and storm is the most powerful. Reflects the tier 2 idea above, where you are using an external source for the power to do something.
Types of magic
I've never seen something explicit like this in any fictional work, but it may be useful as a framework for your 'types' of magic.
Force magic: It applies forces to objects or between objects. Telekinesis.
Temperature magic: Move thermal energy around. Move heat between the earth's core and the castle yonder. Move cold from an iceberg. Pyrokinesis. Easy to learn. Easy to kill yourself before becoming a tier 2.
Space magic: Establish congruencies between different places. Magic mirrors, portals, teleportation. Clairvoyance. Eternal fountains (a small portal to a lake in the mountains. A skilled mage can handle the change in energy/momentum with the transition. A beginner mage better be cautious about teleporting more than a few miles.
This means that you could dig your basement either hiring a force mage or a heavy duty space mage. (In my mental view, a force mage is at the 'common labourer' level of mages.)
Mind magic: Guesting, as mentioned above, reading minds, emotions, speaking mentally, pushing emotions into someone else, transferring emotions. Removing someones mind from one body and putting it in another with or without consent. Reading is easier than 'writing'. Emotions are easier than thoughts. Taking over someone's body against their will is heavy dark magic. Even this has levels of skill. (Consider the difference between a skilled bobcat operator and a new person starting. A new 'possessor' will have jerky control. And any person the first time in another body will be clumsy. Taking over Hitler's secretary and making her slit Hitler's throat with a letter opener isn't in the cards.)
A mind mage may be able to either teach a spell or cast a spell that shields someone else from a less powerful mind mage.
Time Magic Reading tea leaves, crystal ball, cards etc. Prescience -- seeing the future, the past. Time travel, forward, backward, alternate time lines. If you have no alternate time lines, then generally I would expect reading the future to be harder than reading the past. Future readings may be forced to be either unchangable or ambiguous. A prediction "You will die tomorrow" is certain. So a vision of peace and plenty is fine, but if the recipient of the prediction is able to take meaningful action against the prediction, it may be that the forecast becomes vague -- the crystal ball fills with fog. Even this can be useful information -- it means you have to be on guard.
Death Magic Restoring life, communicate with the dead, using a spirit guide. Some aspects of this become needed for all forms of magic if your magic system is powered by life force. Bringing someone back to life may require the death of someone else. Everlasting youth is an example of death magic. I stay young, but someone ages twice as fast. Can the other someone be a horse? A cow? How about mis-using the Eternity Spell to make a victim last longer under torture.
Some of these have obvious sub categories depending on the energy required. I would expect the ability to see the other side of a card to be easier than to read a book in an underground dungeon. Some people may have the power to 'move their point of view' to a given location, but only see what a real eye would see from there. Others may have a more detailed "sense of perception" that can see a 3D object as a whole, and thus can easily read a closed book.
I would expect reading a persons emotional state to be easier than their thoughts. And that reading is much lower effort than writing. (Compare radio -- reception is dirt simple compared to transmitting)
You may have mages with each specialty. It could be that someone capable of mind magic doesn't do forces. Each skill takes time to develop. It takes a very practiced mind magician to steal someone's mind and put it into an 80 year old man dying of cancer.
Most mages can do basics of several fields. E.g. Any mage can use a lux spell, heating up a small ball of air and keeping it glowing.
Persistence and canned spells
Some spells are persistent. E.g. a space magic between two pieces of glass allows one to see the other's light. So having pairs of glasses on opposite sides of the world mean long term street lamps. Do a portal once behind a valve and section of pipe, with the other end in a high mountain lake on the other side of the world and you have a water cannon, with the water coming out at about mach 2.
Spells can be canned, and released with the magic word. Some have to be released by a trained mage, some can be released by ordinary people. "Mirror mirror on the wall, show me Cindy's Mirror" An object like Tolkein's Palantir has to have a strong mind to control where it sees.
Good vs Evil
Randall Garrett's novels have black sorcery as being destructive to the practitioner. Good is partly a matter of intent, but also of side effects. A mage who uses human blood magic to save other people may have his mind destroyed by doing so. Much like some of the physicists who built the atomic bombs, but more so.
In this case someone who is a true sociopath can last longer, but none-the-less is destroyed. Can you have magic that protects your mind from the use of black magic? Your world... Maybe it's a hard spell, and most people are destroyed testing it.
Law of similarity: If two objects are sufficiently similar then acts done on one affect the other. This is a form of space magic. Voodoo dolls are a classic example.
Law of congruity: Two things that have been in contact in a relevant way in the past, have a bond that continues into the future. E.g. the button on a dress can be traced back to the original dress. Could be either space or time magic. Hence you need hair or fingernail clippings to link your voodoo doll.
Law of Relevance: These effects are increased the more relevant the parts are to each other. The bullet is more relevant to the gun, than to the victim. But the trigger is even more relevant.