I'm going to mention the circumstances and surroundings a way to determine spell power. The presence of blood is one particular element in the environment, but each ritual/type of effect/spell could have a list of elements that facilitate it: examples of elements that could matter: material types, colors, presence of creatures, personality characteristics, furniture, height differences, terrain features like large height differences, belief in particular deities, imagery, types of clothing, time of day, moon cycle, ebb/flood status, weather, etc., etc.
In order not to make this look like random shopping lists (not more than is fun, anyway), there probably should be general rules, like:
- summoning is facilitated by images of the creature you summon
- crystals facilitate all scrying/identification spells
- the concentration trait of the caster always contributes to the spell casted
- healing magic is aided by soothing music
- ...
These could be in the raws, while the other elements of the spells could be randomized from world to world, depending on the available elements in that world etc. It also offers a way to learn a spell and improve it later ( = learn more contributing elements).
A concrete example: summon black-crested gibbon has ten randomized elements (orthoclase, peach-faced lovebirds, strawberry wine, blue, the god As, scarves, brooks, tin, noon). A setup conductive to summoning gibbons would then be an orthoclase building next to or over a brook, engraved with images of gibbons and adorned with peach-faced lovebirds in tin cages, while the casting would be done by mage that worshipped the god As, dressed in a blue scarf and standing on a barrel of strawberry wine.
(Another option is to only have general spells (eg. create energy, summon creature, ...) and let the specific form and result of the spell depend on the circumstances. Eg: if you summon a creature amongst ivory statues of elephants, you'll likely get an elephant. If you create energy, it might show up as an aura, bolt, wall, or uncontrolled, depending on the random associations in worldgen.)
The importance of linking it to the surroundings has advantages:
- It encourages the manipulation of the environment, something that's particularly suited for dwarf fortress.
- There is a defensive advantage. This in turn preserves more diversity in the world, making adventuring and trade more interesting. In addition, it's an incentive for trade and adventuring to obtain exotic items.
- It allows a player to specialize in particular spells, but it's practically impossible to have an optimal location for every spell in a single fortress. Likewise, it's very unlikely that an adventurer can use his magical power as a generic (and therefore boring) solution to all problems.
- Different fortress locations make for different magic options. Different locations make for different adventuring atmospheres.
(Also, it's not a battery. Most games make powerful magic and then try to limit its unbalancing effect by limiting the amount of castings, the casting energy or the amount of charges in a magic item. That's too technological for magic, IMO.)