The division of fighting units into nobles and "hirelings" makes this
somewhat troublesome. Can you target a spell directly at a noble or
does it simply fry some hirelings? Targetting nobles directly seems a
bit severe so I would avoid it.
Next, since dying is pretty boolean for all but nobles, you get into
the issue of comparing dragons and peasants. "Instant kill" magic
should also be out.
That leaves you with the combat rules you already have...
A possibility is a series of "blast" type spells that do "<n> strenght
<x> attacks" when cast. For example:
Fireball: 2 mana Does 4 strength 20 attacks
Lightning Bolt: 1 mana Does 1 strength 80 attack
You could also add some special effects making certain spells do more
effective attacks against certain types of units or defeat the special
ability of Holy Warriors. There is a lot of room to play around here.
The question is then how to use these. Does a mage cast these up
front, preparing for battle, or are they cast on-the-fly via some
sort of flag? I would suggest a preparatory casting.
>But perhaps this is not the case. I can envision some simple
>attack spells which could fit into the existing system and not
>be too unbalancing....
If you do add "Combat Magic", it should be a new area. That would
also allow you to append it onto the existing game. It should also
only be taught in a few places, IMHO. Think fast before all the
cities are visited... :-)