A lot of the ideas for magic floating around out there on the forums feels tacked-on, more like an afterthought rather than an integral part of the game. It extends the bounds of the game arbitrarily, failing to utilize much of the game's pre-existing game architecture.
(I am dreadfully sorry for the wall of text that is to follow, I've been building on this for quite some days now)
Magic can Change Temperatures:
Magic should encompass the raising and lowering of in-game temperature (not sure how this would work out with temperature off). I mean by direct raising/lowering. Not only of tiles, but also of individual items and creatures. Imagine the mage-dwarf Urist McMage is attacking an elephant. Urist increases the temperature of the elephant to 25000, instantly lighting it on fire. Hilarity ensues as the burning elephant tries to chase down the evading Urist. Or, imagine Urist needs to cross a river, but can't swim. Urist lowers the temperature of a large portion of the river tiles in front of him to 5000, freezing the water. Urist waltzes across, as the burning elephant melts the ice and begins drowning.
"Temperature change" should also encompass some things that don't use temperature in-game, but are implied to use temperature, e.g. forges, smelters. A fortress with a good mage could save a lot of their fuel supply if they do not have magma (though yes, the mage's magic would be limited).
The maximum amount of temperature change should be determined by the Mage's skill. A more highly skilled magic-wielder could obviously make a bigger temperature change than a less-skilled one.
Magic can Control Object Velocities
Take Urist McMage again, this time being chased down by a Markself for burning down her forest retreat. The Markself fires an arrow at Urist - surely this is instant death, is it not? It isn't. Urist stops the arrow in mid-air, setting it's velocity to zero. Then, Urist throws the Markself off the nearest cliff. A few frames later the purple text 'Markself (name) has died after colliding with an obstacle' appears. However, Urist isn't done yet. A swordelf has been pursuing for some time, and has finally caught up. Urist pulls the sword off the elf and then levitates it towards himself, catching it. Urist lights it on fire using his temperature-control abilities, after which he proceeds to lop a defenseless swordelf's head off with a flaming sword.
From what I can see, velocities are an on/off thing; either an object is moving, or it is not moving. A mage could control whether an object is moving or not, and change the direction the object is moving in. Once again, the mage's skill would determine how successful they would be at changing something's velocity, as well as other factors about the object. Deflecting a light wooden arrow should be simple, but deflecting a steel ballista bolt is a whole other matter.
Magic can Control Flows
Urist McMage, after holding a flaming sword is now... On fire, surprisingly enough. There is a pond nearby, but Urist cannot swim. Surely he'd drown. Thankfully though, he still has a bit of magic left. He causes some of the water to flow upwards, splashing around the edge of the lake, putting out the fire.
After traveling for some time, Urist is assaulted by a band of wolves near a big river. Tired of lighting everything he sees on fire, he decides to try something new. He draws water from a nearby river and forces it towards the wolves. The water knocks the wolves off a conveniently placed cliff.
Such a thing is possible with the current flow system, IIRC, if the flow could be controlled.
Magic can Conjure Items and Creatures
Urist is really hungry, so he conjures up some cat tallow roast to feed himself. Obviously, conjuring is really limited - it cannot be done too often, it will always be of no quality, and it counts as imported. However, it does have some uses - conjuring some water to freeze into ice, which can be thrown at opponents; or to drink. Conjuring sand in fortress mode to have glass anywhere you'd like. Conjure a piece of coal, set it on fire, and throw it at an opponent; instant fireball.
Magic can Change Certain Raw Tags and Gameplay Constants (For specific entities)
Let's take an elf this time, a druid. Her people have been starving for some time, so she causes the prickle berries in the elves' farming plots grow faster, halving their Grow Duration. Then, she makes them more bountiful, doubling the stack size. Her people have also been somewhat short of wood, so she shortens the growing time of the saplings nearby from 3 years to 3 months. Other things that could be changed - an item's weight, melting/freezing/boiling point; creature speeds (a 'haste' spell), size ('grow' spell?), layering, damblock, homeotherm, etc...
As for an implementation of these ideas, I'm sort of stuck.