This sounds like a really great idea. I've never really been satisfied with the magic system in FFH: it's very rigid, relies heavily on memorisation (once you have the basic concept of mage promotions down it's just about knowing what tier 2/3 spells there are and which ones you want), makes it rarely worthwhile to focus on a single mana type or small group of mana types as opposed to getting 1 of everything useful... And adding new spells to the existing system is a matter of swapping out for existing spells with no real room for unique/multimana/etc. spells. The ability to just add a spell one off, with a variety of potential limitations and restrictions, would be HUGE for modmods. Heck, if I could do most of the work to create a new spell by adding a new promotion, *I* might even get off my lazy arse and modmod FF 
There are two things I don't see a lot of discussion of so far that merit serious consideration: balance and AI.
As for balance... Many spells are core parts of either offence or building. The terraforming spells, for example, are crucial to some civs (sidenote - terraforming could seem less like an accidental feature now, with its own spell branch and promo line instead of the pieces to a terraformer being scattered across spell spheres with no indication to a non-FFH vet where they are or how to use them), and obviously combat spells play a big part of the game. You'll have to look very carefully at how you redesign spells like Fireball or Scorch within the new system - any significant change in when they come around or what techs/mana/etc. a civ needs to get there should be considered and tested carefully.
Onto AI. The AI sucks at using magic, it's no secret. Without going on and on on a topic I know little about, I'll just ask: any idea how these changes might affect the AI's ability to use magic, or be taught to use magic in the future? I don't have a clue how the AI deals with promos.
Love the idea, I've thought of similar changes to make the magic system less rigid and move it closer to other games' approaches but your idea is a lot more concrete and thought out than any of mine ever were. I hope you manage to create this so I can play it!

There are two things I don't see a lot of discussion of so far that merit serious consideration: balance and AI.
As for balance... Many spells are core parts of either offence or building. The terraforming spells, for example, are crucial to some civs (sidenote - terraforming could seem less like an accidental feature now, with its own spell branch and promo line instead of the pieces to a terraformer being scattered across spell spheres with no indication to a non-FFH vet where they are or how to use them), and obviously combat spells play a big part of the game. You'll have to look very carefully at how you redesign spells like Fireball or Scorch within the new system - any significant change in when they come around or what techs/mana/etc. a civ needs to get there should be considered and tested carefully.
Onto AI. The AI sucks at using magic, it's no secret. Without going on and on on a topic I know little about, I'll just ask: any idea how these changes might affect the AI's ability to use magic, or be taught to use magic in the future? I don't have a clue how the AI deals with promos.
Love the idea, I've thought of similar changes to make the magic system less rigid and move it closer to other games' approaches but your idea is a lot more concrete and thought out than any of mine ever were. I hope you manage to create this so I can play it!
