Kinematics
Hero
I like the OP's idea. I'd probably cost each spell at the cost of buying a spell slot, rather than the return from selling a spell slot, and I would also let the spell last all day (or at least til the next short rest). But overall it seems like a good way to trade sorcery points for spell flexibility, particularly when metamagic is mostly useless to the character (as it often is for mine).
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Personally, I'd prefer sorcerer's "thing" to be true metamagic, rather than the junkpile that they were given. That is, be able to fully craft spells on the fly, à la Mage: The Ascension, or Ars Magica. Keep it restricted to one or two narrowly focused elements or concepts, and leave it to the player to figure out creative ways to use that power.
For example: You can use cold magic. Don't worry about the crap options in the D&D spell list, just figure out how you'd use ice to accomplish your goals. Whether that's a slick patch of ice, an icicle through the heart, freezing the enemy to the floor, condensing water in the air to create fog, creating a bridge of ice over a chasm, or whatever else — be creative with what your theme can actually do. On the flip side, you're never casting Fireball, or Enhance Ability, or Comprehend Languages, or Haste, or Wish, or Charm Person, or dozens of other spells. That's just outside your capacity. That is what I really expect out of a sorcerer.
However D&D just isn't designed or balanced to be able to handle that. I tried looking at what would be necessary to fit the idea in with D&D's mechanics, and it's just nightmarish. There's not really even any decent math for properly balanced spells, never mind casting stuff on the fly. Maybe, at best, just describe the spell, and have the GM say, "OK. Gimme X sorcery points for that. Here's what happens..." You're entirely dependent on the GM's whims, and the GM has to be able to judge things on the fly (and probably needs experience in open magic game systems), but that's about as close as you're likely to get. ... Unless you literally curate and reskin an exact list of spells that you're allowed to use with any given origin, which might almost be workable, maybe.
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Personally, I'd prefer sorcerer's "thing" to be true metamagic, rather than the junkpile that they were given. That is, be able to fully craft spells on the fly, à la Mage: The Ascension, or Ars Magica. Keep it restricted to one or two narrowly focused elements or concepts, and leave it to the player to figure out creative ways to use that power.
For example: You can use cold magic. Don't worry about the crap options in the D&D spell list, just figure out how you'd use ice to accomplish your goals. Whether that's a slick patch of ice, an icicle through the heart, freezing the enemy to the floor, condensing water in the air to create fog, creating a bridge of ice over a chasm, or whatever else — be creative with what your theme can actually do. On the flip side, you're never casting Fireball, or Enhance Ability, or Comprehend Languages, or Haste, or Wish, or Charm Person, or dozens of other spells. That's just outside your capacity. That is what I really expect out of a sorcerer.
However D&D just isn't designed or balanced to be able to handle that. I tried looking at what would be necessary to fit the idea in with D&D's mechanics, and it's just nightmarish. There's not really even any decent math for properly balanced spells, never mind casting stuff on the fly. Maybe, at best, just describe the spell, and have the GM say, "OK. Gimme X sorcery points for that. Here's what happens..." You're entirely dependent on the GM's whims, and the GM has to be able to judge things on the fly (and probably needs experience in open magic game systems), but that's about as close as you're likely to get. ... Unless you literally curate and reskin an exact list of spells that you're allowed to use with any given origin, which might almost be workable, maybe.