I must admit, I'm a little worried by this idea of a procedural magic system... Magic has to have some level of structure or it becomes useless. The last thing you want is to spend ages training a magic user only to find out that magic consists of throwing a puff of steam a few squares.
You'll know what's possible, mythgen prototype includes descriptions for each race with what they can do as far as magic goes and it's cost. Toady's talked about specifically hiding the deeper secrets of the universe, not the standard.
Hiding what the non-playable civs of a particular world are capable of might be kind of Fun though...
--
Imagine that. No real idea of what the enemy may be capable of until the first siege.
Fireproof walls - check
Traps in possible tunnel locations - check
Adamantine bolt throwers - check
And here comes the invasion!
[Pixies are capable of teleportation with an occasional chance of wide area effect self-destruction each time]
Noooo!
Fun is had.
Figured out your strategy against all possible enemies after playing a few fortresses? Smite the world with Armok's hammer and start again.
Eventually all possibilities are figured out, at which point a new release hits with an additional layer of procedural magic possibilities and the Fun starts over again.
Plus mods, adventurer, etc, etc...
No, I think procedural magic has the potential to be awesome.