How did you calculate the DC for the spellcasting check? i.e. 10 + spell level +1 because spell school in opposition?
And which spells being cast were isolated that increased the DC? Why not just increase their spell level?
So I didn't actually do the proposed Spellcasting Nerfs when I ran 5e. I just did the 6 listed bullet points at the top.
The 7 bullet points at the bottom (including Spellcasting changes) are what I proposed would need to be engineered (and stress-tested...and then inevitably iterated upon) in order to optimize 5e for Story Now play.
If I were going to undertake this project, I would probably begin with something like:
* The spellcasting check vs DC has a spread of 40-50 % Success rate with Success With Twist being 30-40 % and a low % of (just) Twist result. I would have a very mildly graduated relationship between Spell Level and this result spread...probably lump things into Cantrips - Level 2, Level 3 - Level 6, Level 7 - Level 9 buckets with mildly increasing DCs.
I'd try that out first, see how it plays, adjust if results are undesirable (understand..."desirable" here means (a) heavily nerfed spellcasting and (b) much more dynamic decision-space and consequence-space...tactical, strategic, thematic around spellcasting).
I honestly think the basic Fate point economy could be bolted on to almost any system, as long as certain other assumptions are ported in with it.
Your high concept should probably include your species and class, with an extra adjective or so: "Devious Elven Ranger" or "Human Wizard of the Order of the Star".
Invoking an aspect gets you a reroll or a bonus to the dice roughly equal to a standard deviation. (For a d20, that'd be about +6.) In 5e, I'd probably let it eliminate disadvantage if desired instead. Given that in 5e, damage is rolled separately from hitting, you could potentially invoke on damage rolls as well!
Missing the DC of a roll by 1 (or possibly a bit more, maybe 1-3) might give you a Boost.
Along with the economy, it would be highly desirable to import the idea of "success at a cost". And the GM would want to apply aspects pretty freely to NPCs, areas, and scenes.
Done, pretty much. I haven't tried it, but I don't see any reason why it wouldn't work. You could even give people Consequences when they get hurt enough (0 hp, probably - but 1/2 hp might be a Minor), or something dramatically awful happens.