I haven't dealt with magical world in a while, but there was an idea I developed years ago. I wanted to write a few stories in this world but never really got the chance... Still in my pile of ideas that I might come back to someday.

The world is a dual world (at a spiritual or physical level, I don't know yet): two worlds interconnected magically.

Magic is a form of energy from which life originates (something something abiogenesis via magical energy). Some individuals (human or not) have the ability to channel magical energy from the environment through themselves, and then transform or manifest it in the real world. This has on inherent drawback on the user any worse than physical exertion: throwing boulders magically or physically will tire one out physically and mentally and that's it (obviously depending on physical and magical strength, technique and training). Many magic users learn to convert it into only one form or work with only one type of medium, from which comes the idea of elemental magic: manipulating heat to induce combustion or control kinetic energy of liquids. Elemental magic has a historical importance in both worlds, but is generally regarded as different ways of using magic now in the magically dominant world. There may be a basis of natural ability (genetic or otherwise) for magical specialisation.

Magical energy obeys conservation of energy, where each of the dual worlds is a closed system. If you're heating a litre of water up from room temp to boiling, you'll need to channel 75kJ of magic into heat (probably more, maybe 2x or more, given channelling losses - humans aren't very efficient energy converters - plus losses transferring it into the water).

Magic also obeys another conservation law, this time a transworld one, and this one is key to the entire history of the two worlds. Magic that is converted in one world continues to exist in that world in its new form, but at the same time, equivalent magical energy is created in the world's magical dual. This implies that there is a constant amount of magical energy between both worlds and it forms a closed system in terms of magical conservation. Also recall conservation of energy within each world: this creation of magical energy manifests as the spontaneous conversion of other energy into magic (usually heat - haven't thought through the consequences of that too much aside the obvious feeling a random chill, I had chosen this because heat tends to be an entropic dead end for energy).

A fixed amount of magical energy in both worlds implies that the concentration of magic in each world fluctuates in a complementary fashion. This has generally created a cycle that lasts around 10,000 to 50,000 years: long enough for entire civilizations to live and die and be entirely forgotten, and magic to become myth if any memory of it is retained at all.

Human civilizations in both worlds have a much longer history than ours. It's typically characterised by a rise in magical energy causing several civilizations to do rediscover magic. Destructive energy being much simpler to harness than constructive, those civilizations that focus on military magical forces quickly gain dominance of the world and grow into powerful empires. The discovery of magic and formation of these empires generally squashes any technological innovation and reverts technology to a certain extent.

War generally dominates between these empires for a few thousand years, fuelled by the corruption that a sudden influx of magic brings. Peacetime will often appear here and there, allowing constructive magic to develop and technology that harnesses magic. At some point, a war grow in scale, the military mages grow in power and technological enhancement, to the point of reversing magical energy flow. These continued wars quickly deplete magic (usually the other world has depleted to the point of not counterbalancing the magical flow at all long before that), causing a sudden collapse over a few hundred to a thousand years in many civilizations that became magically dependent.

Civilizations usually fall as some underdog (or group thereof) retaliate with nonmagical methods or strong strategy. This generally brings about new civilizations that evolve much as the ones we know... Meanwhile, the other world has gained most of the magical energy back and is in the initial phases of this cycle.

The story I was thinking of writing takes place at a point where magic is depleting in a war between two advanced superpowers, while the other world is in an technologically advanced, relatively peaceful world with magic slowly reappearing. One girl from the first world, a magical acolyte from a colonised nation, is found in the second world with only fuzzy memories of her past...