Variant Magic Rules
Spell list is available here, to determine what kind of magic is being cast. Bards can cast as nature magic and warlocks can cast as divine magic if those options are available for a spell on their spell list, even if it is also classified as arcane. Otherwise, if multiple spell types are listed, assume all casting aligns with your class. (Ranger and druid spells are nature-based; sorcerer, wizard, artificer, and most spells gained from magical subclasses are arcane-based; cleric and paladin are divine-based.)
Works as normal. As far as you know. :D
In the long absence of other magics, and following the collapse of several civilizations that allowed nature to reconquer large parts of the world, nature magic has become stronger than it ever was before, and by far the most recognized and relied upon kind of magic in this world.
When casting nature magic, roll 1d10 – before rolling attack, if it is a spell that requires an attack. On a 10, choose one aspect of the spell (damage, range, duration) and double it. Alternatively, if the spell requires an attack roll that fails, you may reroll once and use the second result instead.
Potions are made using natural magic, and so work as normal. (They do not receive the benefit of doubled effects the way spells do.)
Arcane magic is broken. The reason for this is not widely known, but everyone agrees that yeah, it sure is broken. And scary. Wizards and warlocks are regarded with heavy suspicion or outright hostility — anyone willing to mess with that volatile a force is probably mad or just doesn't care what kind of damage they do. Sorcerers usually receive a bit more sympathy, but they're still likely to be seen out of town with torches and pitchforks when discovered, if they don't leave town willingly — they're far too dangerous to be around normal folk.
When casting any arcane magic, roll 1d10. On a 1, the spell fails. On a 10, the DM will roll on the Fuck You Table.
When casting with a spell slot of 1 or higher, roll an additional d10 for each spell level you are using. If you get a 1 or 10 on any of the dice, apply the appropriate results. These results do not stack (you don't have to roll multiple times on the Fuck You Table, because I am merciful), but both can happen at once.