Similarly, as the four elements are what certain areas of knowledge on our planet and culture seem to tell that humans consist of, so could well the bodies of the mana world inhabitants consist of.
In such case, the only 'magic', as it would logically come from within the player's existance, would have to come from these four elements, or the combination of them.
In such case, I suppose having four differrent shrines/cults available, all having different schools and quests, allowing players to further on their knowledge of a particular shrine, that is, Shrine of Earth, Shrine of Fire (or Sun), Shrine of Air (or ether) and Shrine of Water.
Each town/place could also naturally have some or many shrines, or shades of shrines, bigger/better or smaller shrines, etc.
Some shrines could exist in more pure forms out in the wilds, such as closer to some rivers/waterfalls (the water shrines) or closer to lava/cave systems and deserts (the fire/sun shrines).
In these shrines, one could further their abilities in the growth of a particular kind of 'magical' arts - that is, arts of water, arts of earth, arts of air and arts of fire.
In these arts then, composed in particular ways and at particular levels, certain spells would be produced, allowing therefore for, apart from having some basic spells such as 'set fire', or 'change water state' or 'alter earth structure' which would come at basic levels of these arts obtained, such as lv 1 of fire arts would give us ability to set small fires, to have personal spells made and invented by the particular player.
I propose having mastered lv1 of
Water Skills - would give the player the ability to alter water state (solid/fluid/gaseous).
Air Arts - would give the player the ability to cause a disturbance in the air flow (sudden wind blow etc).
Fire/Sun Mastery - would give the player ability to concentrate the power of mana from within to start small fires on simple things (like paper).
Earth School - would give the player the ability to shatter small blocks of clay (could be use as offensive ability later on, to say concentrate this with an attack, passively with level growth, to cause more damage during attacks with particular melee weapons).
Lv2 water + lv1 air + lv1 sun/fire + lv1 earth would then give the player ability to master the flow of mana throughout the body and therefore perform simple healing.
Etc.... the possibilites are endless, although some engine would have to be written behind all this to allow creation of own spells, and letting them grow.
Having players to create own spells would be interesting, and naturally pushing them to further studies in particular schools to empower chosen spells and levels to create even better personally created spells, would allow also for greater customization of a character - I think the last thing we'd be aiming for is standardizing every character and his/hers abilities to the point of dullness.
These certain schools could also come at a natural cost, that meaning, having for example pushed fire arts to an extent where pushing water or air arts would start being limited, or some skills such as 'pick pocket' or 'craft weapon' or so would also be limited due to the nature of fire growing throughout the character.
Let us have a brain storm on this, and so far it is also clear to implement an INTERESTING and powerful magic system would require other skills to be in place, so let's put our minds to work
I do say this bearing in my mind by all means the thought that my here presented system is not necesarily the best and only, but I also think it's a start and a valid suggestion and as that, if it were to be liked, I guess it would draw a nice beginning
We definetly need skills apart from it, had the two been to incorporate and interact with each-other, and I believe starting anything to make it mediocre won't even make it half-decent.
The above ideas also are mostly server ideas.
The client side could probably need some work too to show the effects of the spells
Cheers.
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EDIT: as stated here: http://wiki.themanaworld.org/index.php/Backstory there already even is a mention of the four godlets, so, those each could also impersonate the earth/soil,fire,water,air..