Wow, I'm just full of this kind of thing lately...
Alright, here's the basic idea:
1) Every spellcaster has a Mana Capacity
; this is the maximum size of their Stack.
2) The Stack in this case is literally
a stack of cards. This will be important later.
3) Mana cards will have different types; think MtG. A spell's cost is paid by removing the indicated number of Mana Cards from the top of the Stack.
4) All spells will cost a variable amount of Mana; any spell can be cast with most kinds of Mana by default.
5) Some spells might have a list of opposed
Mana types; those Mana types cannot be used to cast that spell, and can't be included in their casting.
6) Some spells might have a list of attuned
Mana types; those Mana types give a boost to the spell in question.
7) A beginning spellcaster starts out with "blank" Mana cards up to their full capacity.
8) After each rest (variable depending on system), the Mana Stack is refilled to capacity with blank Mana, and then shuffled.
9) The place that you rest affects your Mana recovery; a location has either a minor
aspect or a major
A) A location that has a minor Aspect replaces 1 of the Mana regained with Mana of that type; sleeping in a graveyard might give you 1 Death Mana, while sleeping in a forest that just burned down might give you 1 Fire Mana.
B) A location that has a major Aspect replaces all of the blank Mana with Mana of that type; sleeping in the Underworld would give you a mess of Death Mana.
C) A location can only have on major Aspect, but can have multiple minor Aspects, or a mix of one major Aspect and multiple minor Aspects.
10) Any spellcaster can spend an action to shuffle their Mana Stack. They can also spend an action to "burn off" the top card of their Stack.
11) There are two special Mana types: Null
; Null Mana is opposed to every spell, and Supernal Mana is attuned to every spell.
This system has some things that are, in my mind, an interesting way of affecting spellcasting behavior. Imagine a deathly realm that has a Major Death Aspect and a Minor Null Aspect; most spellcasters will want any adventure there to be a quick in-and-out, since it'll take caution to stop their Stack from being cluttered up with useless Null Mana.
They'll want to be out of a realm that just has a Major Null Aspect ASAP; unless they have a way to "bring magic with them", they'll run out sooner or later...
Oh, and mage-slayers will obviously have a way to clutter up an enemy spellcaster's Stack with Null Mana... which'll be less than effective against a dedicated Pyromancer, who might have an ability that puts one Fire Mana on top of their Stack whenever they "burn off" the top card of their Stack.
Thoughts be welcome.