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Yeah, I'd probably play in something like that.
Yeah I was definitely thinking along the lines of a sphere/element system. Say you could be an earth mage, but your specialty lies in rock/magma/dirt/plant ect. So that your core spells revolve around one aspect of the larger sphere. Though you still have access to weaker versions of the overall sphere, and with training possibly some basic spells from connected spheres. Like say, you are an earth mage specializing in rock, you could bleed over into magma which would afford you some knowledge of the fire sphere, as long as it's related to molten rock.
Do you think it would work with the concept to have different mages using different methods to cast spells? There was one game in the main FG&RP section where a mage could either cast spells by writing runes or using martial arts or swords or capes. It could have been potentially really cool, but he made it into a "You are..." and combined the top suggestions for elements and casting methods to make a rune writer who cast air spells, which no one was happy with. But I think the concept would work rather well in a game where everyone got to make their own character.
In unrelated news, I came up with a cool idea for a magic system, which probably wouldn't work for this but would be its own thing, based on what I likely misunderstand of the Australian Aboriginal concept of Dreamtime.
So the way this works is, magic is a Gift, in a literal sense. Anyone who has magic has one or several powers or spells that they can freely give to whoever they please or trade it if they want. It's almost like a metaphysical item that can only exist in human (or demihuman, if they exist in the setting) bodies, needing close proximity to jump between bodies in an exchange. The one passing on a Gift doesn't even have to be alive to give it; if they will it to someone, that person can just stand near their remains for the exchange to happen.
Maybe there might be a way store a Gift outside of a body, probably only used for non-barter trade, since in that case anyone who has the thing that holds the power can just take it without its former owner's permission. It's much safer and easier, otherwise, to just transfer a Gift to a person, because thieves can't take gifts from bodies, dead or alive (this will be true in at least general sense, though probably not in an absolute one; there probably would be at least one thief in the world who is so skilled their craft that they can steal Gifts, but this would be a unique exception to the rule). Now I'm thinking professional merchants would need to either use these Gift storing items or hire people as holding bodies, as there would be a limit to how many Gifts one person can hold; either because so much power would be overwhelming or several gifts would come with certain taboos and restrictions attached to then that couldn't be followed simultaneously.
Now the reason these are called Gifts is that they were originally given to people by various powerful beings. So, for example, in the time before time, someone's great great great ect. grandfather helped some sort of spider spirit/god and she, in gratitude, gave him the Gifts of wall-climbing and talking to spiders. Then, somewhere down the line, another one of his ancestors catches and spares a magical fish and is given the Gift of water-breathing. And so, eventually, this great great great ect. grandson of these guys has kids, and, on his deathbed, he bequeaths the Gift of talking to spiders to his eldest, spider-climbing to the middle child, and water-breathing to the youngest, because they live in a desert now and youngest aren't supposed to get inherit anything that seems immediately useful.
And different families might have more or less Gifts available to them depending on how often they've come into contact with magic spirit animals. The Gifts given might also be more or less useful than these, depending on where they come from: there are some sprites that will give you the Gift to jump just a little bit higher if you just say hello to them; these make up the bulk of traded gifts. There are also some gifts that can either only be used a certain number of times before they're used up or ones that can only be used something like once a year or more.
I think this would be a pretty neat system, though I am worried about what would keep players from willing their powers to each other. I had considered the idea that only the first players would be given Gifts to start with, which seems unfair to later players. Alternatively, players might either start out with no Gift and have to obtain them in game, or they might have to roll on a chart to find out what gift, if any, they get. I'm not too happy with either of those ideas, but they're the best I can come up with right now.