And once again, the example of what "wild magic" means comes down to "it has a 20% chance of killing everyone nearby." Whenever I ask what "wild magic" means in terms of something that can actually be in a game, it always comes down to "it kills your dwarves if you use it". This isn't complex behavior, it's just a random game over if you're stupid enough to use the system.
Again, DF is fundamentally a game of managed risk. ...
Let me try and give a different sort of example. Let's assume for the moment that future!DF has a sliding "chaosity of magic" parameter; and that it can reasonably be condensed into a simple parameter of small integers. (Given the "fantasy level" parameter for the mythology generator shown in the GDC talk which works exactly like that, I think it's a decent starting point at least.)
Due to whatever combination of origin myths (which seem to frequently involve cosmic eggs, and commonly animal deities), let's say there is a spell that creates turkey eggs; as this is nominally for your breakfast omelette (Turkey Egg Stew: Expertly Diced Turkey Eggs x3), the simplest version creates 3. (The costs, difficulty in researching or learning, etc. would be set elsewhere in the generator.)
Chaos 0: Spell creates 3 turkey eggs (3-3, avg. 3)
Chaos 1: Spell creates 1d3+1 turkey eggs (2-4, avg. 3)
Chaos 2: Spell creates 1d5 turkey eggs (1-5, avg. 3)
Chaos 3: Spell creates 1d6-1 turkey eggs, with the dice
exploding on 6 (0 to infinite, or maxint at least; but still avg. 3)
Chaos 4: Spell creates 1d6-1 items, with the dice exploding on 6 (0 to infinite, or maxint at least; but still avg. 3); plus an additional d6 is rolled... 1: a random sort of egg; 2-5, turkey egg(s) as expected; 6: a random turkey product (varying the kind and the type at the two ends; a spell to create gold ingots might have a chance of random ingots, or random gold things)
Chaos 5: mostly as above, but the additional die is 1: random source or predecessor; 2: random egg; 3-4: turkey egg; 5: random turkey product; 6: random target or successor. You might end up with a live turkey, a ready to eat 3x Turkey Egg Stew (omelette, more or less), or something along those lines. The gold ingots spell might give gold ore, or gold figurines.
Chaos 6: adds another die, which on a 6 gives a random modifier or state to the result so far... eggs might be fertile or rotten or boiling, nominally-live turkeys might be giant or starving or husked. Gold ingots might arrive molten, or as masterwork studding on everything in the workshop.
All of the above are based on fairly straightforward algorithmic modifications of the initial "predictable" result (3 turkey eggs), plus trawling around in pre-existing relationships in the raws. No one would argue that even the Chaos 3 version *can* be a lot less predictable; but really you've got less than 0.5% chance of more than a dozen eggs, the *average* remains 3, and if some fort somewhere generates four dozen eggs from a single casting, that's amusing while also being one in a million.
Even stepping up to Chaos 5... getting several live turkeys when you asked for 3 eggs is certainly a bit surprising, but in the general scheme of fort (or adventurer) life is more likely to cause an amusing anecdote than the fall of civilization.
Only when you turn it all the way up to Chaos 6 does there become any real potential for disaster, and the odds are very low. Even most of the weird results are annoying and/or humorous; ending up with 11 rotting Turkey Egg Stew in your magic workshop, with attendant miasma until cleaned up, is unlikely to end any reasonably prepared fortress. There's a tiny chance of ending up with a few angry zombie turkeys, or were-turkeys; this could cause a moderate amount of damage to a good fort or end a teetering one, but the chances for this sort of ending are down there with "embarked someplace with undead eagles and rain that stuns the living". And remember that this is a *voluntary* parameter turned (in this example) up as far as it could go.
While the example is a bit contrived, hopefully it helps explain how a simple spell "creates about 3 turkey eggs" can become gradually more chaotic, via entirely procedural means that can readily be programmed within the existing structures.