I'm sure toady would come up with something anyway, he does that kind of thing. Like he said in the alternate planes df talk about an artifact that slowly brings your fort into the fire dimension,
this is also why we would have a slider. some people, like me, like the concept of a world where magic is rare but dangerous and unpredictable, and some like magic being all over the place and unpredictable, creating a dystopian type world.
There are many fantasy settings with chaotic magic, and toady already has a list of magic effects, so he can just use a random number and grab an effect, I don't care if it is always super usefull or not "you turn into a plant" is just fine , since you chose to have your magic chaotic.
Well, simply saying "Toady will think of something" is at least magical
thinking... Again, this isn't a matter of just using some elegant code, I have yet to find anyone who can even
conceptually describe a system that produces magic effects that achieve the sorts of ill-described, nebulous effects you tend to find in fantasy fiction. All they come up with is a Wild Magic Table. It doesn't matter how good a coder Toady is, he can't do the impossible. Judging by what magic interactions he's done, for that matter, vampires and necromancers are entirely "predictable" magic, while werewhatevers are unpredictable only in that instead of being attacked by a werewolf, you instead get attacked by a weresheep.
Fantasy novels have the advantage of withholding information from the reader. Magic effects can be a "surprise" because the reader wasn't clued in to what they do, before, but that effect is rapidly lost when you systemize and make routine that sort of magic interaction. For that matter, fiction novels don't need to keep a simulation of the exact physical effects a spell has upon every rock in such a way that a reader could easily ascertain the exact dimensions to which a fireball spell actually has its area of effect. Harry Potter having hopping chocolate frogs for the first time is a surprise and a "mystery" and novel. When Harry gets on his name-brand flying broom to win his 7th straight quidditch match, they have to introduce villains trying to sabotage or otherwise overshadow the proceedings because flying on brooms chasing after a golden ball that only Harry can catch to always win becomes boring and routine. For that matter, Bertie Bott's Every Flavor Beans where the beans are randomly either caramel corn or vomit-flavored is a really stupid idea outside of the first "sheer novelty" tasting. Even my Harry Potter-obsessed mother and aunt only bought them once before declaring that having a box guaranteed to have flavors like "booger" in it
somewhere if you just kept eating every candy in the box was a really stupid idea.
And yes, it's a slider, and I'm sure there are some people who'd enjoy it so that they could "have their fortress explode epically," which might be good for a laugh the first couple times you turn the air into fire and everyone dies for no reason. I suspect more players are interested in
not barbecuing themselves just as they're getting their fort on its feet just for the giggles, and would weld that slider firmly "off". I'm not sure if it's worth the effort of creating such "chaotic" magic if almost nobody would use it more than once, however.
After all, even when you start talking about it, the
only example you give happens to be a "your dwarf dies for no reason" example. You might as well replicate the effects of this kind of "magic system" by just rolling 1d20 every month, and if you roll a 1, change the raws so that flesh melts at -20 Celcius. Same basic effect as the "chaotic magic" the occasional person clamors for.