Magic should be sphere based. Materials, objects, creatures, plants, images and words should all have sphere tags in their RAW files. Gods and some artifacts also get spheres when they are generated. Signifigant events should have sphere relations. This means that silver daggers, special magically signifigant herbs, and highly specific sacrifices/offerings are used for doing magic.
There should also be magical methods to enchant an object/creature/material with magical power, or bless/curse it so that it gains spheres related to the blessing/curseing deity. This magical sphere info should be part of the "curses" system that will also be used to handle undead and were-animals. Beings with the [POWER] tag should sometimes make bargins with beings in exchange for one of these curses. These curses should also have some other, non-magical effects such as abnormal weaknesses, attribute changes, personality changes and apperance changes.
There should be special languages tied to Powers that have high sphere-to-word ratios, making them more magical. This means people chant in Draconic, Demonic or Titanic when they want to do magical stuff, but not when they don't want magical stuff going on. It also means that when the demon utters a curse in it's gutteral tongue at you, you feel frightened because who knows what it just said and what saying that is going to do to you.
Magic should cause thirst/hunger/tiredness/bad thoughts/wounds. It should also beneifit from a large number of knowledges, such as knowing (and being able to accurately and reliably pronounce) the Power languages. This means that players will want their multi-legendary, ultra-dwarvenly tough, highly educated superdwarfs to learn to do magic, and not Urist McThresher. It also means that specific sorts of magic that have a high "bad thoughts" cost also have a good chance to slowly drive the wizard in question insane if they cast too often or incorrectly. A beserking, ultra-legendary champion-dwarf decked out in artifacts and shouting random magical words at the top of it's lungs in Titanic would be a Bad Thing™ for the fortress.
Magic should be chaotic in the mathmatical sense, that is, being highly sensitive to initial conditions, but with little to no random elements. Words, materials, objects, creatures, positioning, timing, etc. should all be relevent If you have figured out a magical ritual that has a given effect, moving some component of the ritual, changing the timing, changing a single word (or saying it wrong), or altering the initial conditions in some other way should have a mathmatically chaotic butterfly effect on the outcome of the magic. If you preform the ritual exactly the same, however, down to the closest details, then it should have exactly the same effect.
This means that a magic user who has been signifigantly altered in some magical way (ie. granted sphere tags/magic powers by a Power) will see their magic changed and distorted by the changes that occured to them. As an example, if a magic user makes a deal with a demon to gain magical power, but, in doing so, they get the [SPHERE:SUFFERING] tag added to them, their spells will become more painful and/or cause intense suffering in those around them, even when they try to help. Malicious Powers could exploit this fact to further a sphere related agenda, or to satisfy their cruel thirst for irony.
Magic in a world should have a world gen number seperate from the history number and the geography number. This number is used to proceedurally generate the specifics of all magical rituals/spells for that world. This allows people to maintain a consistant set of magical rule between worlds, share knowledge on the forums about it and make magic reletively predictable while still allowing them to gen a new world with a random magic gen number and have everything old be new again.