^
I like the way this is going.
Here's my 2 cents on the matter:
Mana, like Osmosis said, should be a treated as a static flowing thing, similar to liquids, with levels hanging from 0-7. Having specific kinds of magic (IE fireballs, ice storm, etc) is cool and all that, but the great thing about DF is the ammount of influence the player has over the game. As you already can, right now, in the middle of an alpha, channel way, raise it several z levels using crazy thingamajigs of pumps, axles and gears, to make waterfalls, artificial wells and the like, you should be able to manipulate magic in different ways, for a pletora of reasons. Mana could be channeled using runic engravings, certain runes could act as a pump to direct mana to some specific place, in order to attain certain results, a nice example would be channeling magma to your water supply, and with the help of a wizard, use said mana to change all the water into wine! You could also channel mana into you wizard's workshop, increasing his power and range of influence, and helping him learn or maybe even create spells. Making huge engravings in certain shapes (pentagrams and the like) and make crazy things happen, such as creating a portal for summoning creatures (and said creatures could go on a rampage if your glyph wasn't well drawn or your wizard is too much of a n00b), negating gravity to make lifting water through z levels easier, enchanting your forge so the weapons and armor made in it become magical, etc.
Of course, your wizard should also be able to use magic individually, to a lesser extend. Playing a wizard in adventurer mode would require them to at least have some combat spells
, and they would also be important figures in the world gen, creating towers in places with higher magical concentration, summoning demons, etc.
HOW this would be implemented though, i don't know, but i'm eager to see somethingat least similar to this in DF's future.