Can we build the math from the ground up? (And does Vancian help or hurt that?)

[Pex]

Sure, but such as system seems more akin to the Binder's Vestiges or the Cleric's Domains than it does Upcasting. At the point where you're talking about abilities whose only overlap is "does Fire damage", it seems bizarre to insist that these are one ability you power up, rather than merely a suite of abilities.

At this point then it's flavor text semantics. It would then be easier to manage magic access to think in terms of suites of abilities with the game mechanics involved. It's easier to say, write, and think about the Pyromancer gets Fire Attack as his main focus which allows all these things instead of Spellcaster choosing individual spells trying to fit a theme. It then becomes easier to limit access to Magic Forms because there are less overall than number of individual effects.