Just thought I'd revisit this.
Right now, I'm running two HARP games.
In my Mushroom Kingdom game, I'm assuming that all magic is radiated onto the world by the Star Spirits. Every Sphere of magic is its own spellcasting skill. There is the Universal Sphere, which any character can learn-- which includes Universal Magic, Cantrips, and a handful of spells that are more common in the Mario Universe-- and then there are all the different Arcane Spheres, which every Mage and anyone with the Arcane Power talent can learn and learn spells from. These Spheres are Animist, Elementalist, Glamourist, Necromancer, Vivamancer, Battlemage, and Warlock. Spellcasting is verbal and requires the use of a magic scepter (or a -50 penalty) and more powerful scepters act both as PP adders and as bonus items for one or more Spheres.
The most powerful scepters, of course, are the Seven Scepters of Royalty (each one attuned to a different Sphere), the Scepter of the High King (concealed in High Princess Pichi's parasol) and the Star Rod which is an artifact that grants bonuses to all seven. Thinking of giving Bowser his own unique artifact scepter as well.
In my Spelljammer game, I'm using my default house rule. Spellcasting is Verbal and Gestural, but Song and Somatic can be learned as Talents. (Harpers get Song for free.) Universal and Cantrips are one skill, and each Professional Sphere or Circle is its own skill which characters must have the appropriate Arcane Power or Arcane Circle talent to learn the skill and the associated spells. Arcane Dabbler allows any one spell to be learned as if it belonged to any one Sphere, including Universal/Cantrips.
In both games, spells cost 2x their Base PP to learn. (I'm going to warn the players in the latter game before they reach 2nd level.)
There's something else I've been considering, but I'm not going to press ahead with it because it's probably broken. Using College of Magics, there would be three spellcasting skills: Low Magic (Universal Sphere and Cantrips), Middle Magic (all Professional Spheres/Circles, no Arcane Circle Talent), and High Magic. Styles are to be determined. Characters can learn any Low Magic spell plus 1 free Cantrip per rank, any Middle Magic spell they have the appropriate Talents for, and High Magic if they have the appropriate Talent. All spells cost 2x Base PP to learn. Not only is this a little too powerful in my opinion, I think I like the flavor of thematically linked spellcasting skills better.
Has anyone else come up with variants like these for the HARP magic system?