I'm thinking of a character for actual play. He believes that, since everything is made up of the four elements, it should be possible to affect the Forms of Animal, Corpus and Herbam using the elemental Forms. AuAqIgTe requisites should affect any physical thing. Making a breakthrough to allow for this is the goal of his reasearch.
Would you allow this breakthrough? Would it be a single breakthrough, or three seperate ones, one for each Form of An/Co/He? Would it be a Minor or Major breakthrough?
After that breakthrough is done, there are still the Forms Im/Me/Vi that would not be affected by the elemental Forms. Could they be manipulated in a similar way using the elemental requisites under the medieval paradigm?
At least a Major breakthrough I think.
One good starting point for research would be the non-hermetic Elementalists (from Hedge Magic) who can indeed affect both humans and animals using Elemental forms.
I have made at least two magi based on that concept, although neither saga lasted long enough to earn much experience ...
Now, while I think the breakthrough should be possible, I do not think it should be designed to actually play with the reduced portfolio of arts. For me, it would be rewarding to see it developed, see that it is too much, and bring the saga to a close at that point. It matters very little if the magus succeeded in reducing the number of arts by one or by three (or by nine for that matter). The thrill is in the search, and the IC portrayal of the seeker and his philosophical discussions with his sodales.
That implies that I would tune the requirements to last the expected/desired length of the saga. This is not only in terms of the breakthrough thresholds, but also in terms of the insights you make available from other traditions. Deciding on a difficulty in mechanical terms is IMNSHO not the right way to go about it. The task at hand is one of narrative design ...
One breakthrough per art made redundant makes it narratively most interesting. I mean, in real research, one would try to aim for any partial result which can be presented and demonstrated in its own right (a stable form in Herbert Simon's terminology). Solving the problem in its full generality in one go is simply bad project management. This also makes the project more concrete, and one can more easily be more specific in treasures (letters, lab notes, ancient magic insights) along the way.
I do agree with @OneShot that it sounds more like a Hermetic breakthrough than anything less, but canon gives the SG a lot of leeway in adjusting the threshold without being bound by the three canon categories. I would just wing it, based on the reasoning above.
Worth noting that even if such a breakthrough was made, the original arts of Animal, Corpus, Herbam would not really become obsolete.
If what you want is to have a magus specialized in animals, it is a lot easier for him to just study the art of Animal, rather than having to study all four elemental forms.
For a generalist magus it would become redundant to study AnCoHe, since as a generalist they would already have decent scores in the elemental forms, but not everyone is a generalist.
I'm guessing he was gonna take Elemental Magic to go with it to ignore the requisites.
Combining arts is not a new idea, it has been done before in canon I belive. There used to be four different versions of Vim for the realms that got merged and I'm confident that that was not a hermetic breakthrough.
Not four different versions of Vim. Originally Vim was only used for affecting spells, enchantments and the like. It wasn't used at all for the different Realms - for that you used the other forms depending on what kind of phenomena you wanted to affect - Herbam for faeries of the wood, Ignem for spirits of fire, and so on (this is still done by some spells.)
Later on Vim was extended to cover the Realms by Concinatta.
(Legends of Hermes p15)
It can't have been an Hermetic Breakthrough, since we are told that in the canon history of the Order of Hermes there has only been one single Hermetic Breakthrough made - the invention of Parma Magica by Bonisagus.
(HoH:TL p27)
It would depend on te campaign- I routinely come up with, and shift between games, background cosmologies, that 99% of the time my players never realize and could not tell from one game to another that anything was different despite what I consider a radical change in cosmology.
The first question is whether the underlying premise is correct- which ultimately determines whether he can succeed in whole or in part. he is basing his assumption on the classic four elements, but even within western mysticism I have seen schemes wih 5 or 7 elements- if there is an element of spirit for example, that is required to affect living things, then discovering that would be a prerequisite breakthrough. If you utilize the alchemical elements (four standard plus salt, mercury, and sulpher, in which salt represents the body, sulpher the soul and mercury the spirit- other definitions of the associations also exist) then you need a host of breakthroughs that almost defines a new magical tradition.
Or the traditional understanding could simply be wrong on a fundamental matter, that the "elements" which they believe make up all matter are really forms describing the states of inanimate matter, which is far more complex in its actual nature.
Note that in a game I am running some kind of breakthrough in one of these directions (or maybe another one) would be possible- you just wouldn't know where you are going to end up.
What I would do, is allow the player, with one Major Breakthrough per Element (Aq/Au/Ig/Te) to use it to duplicate spells of Co and An, but with two caveats:
Co spells have to be consistent with the appropriate humor of that element.
An spells have to be either consistent with the humor, or apply to an animal that is of that humor.
I would allow a magus with the Elementalist Virtue to achieve with a single Major breakthrough the following Minor Hermetic Virtue:
Greater Elementalist
You must have the Elementalist Major Virtue to take Greater Elementalist. As a Greater Elementalist, you can produce any standard Hermetic Corpus, Herbam, and/or Animal effect at the same level using instead the most appropriate (by humoral theory) elemental Form with the other three elemental Forms as requisites. Your ability to ignore elemental requisites does not apply in this case, as you are already stretching the scope of the elemental Forms far beyond what other Hermetic magi can do.
I think a Major breakthrough is appropriate, in that it does not really break any limit of magic but significantly pushes the envelope. Note that the number of breakthrough points required can always be adjusted by the troupe, and ultimately how many you require is something that depends on the needs of the saga. I don't think it's particularly unbalancing. Sure, it is an advantage, but note that
a) other Hermetic magi can still do everything the inventor can
b) if they specialize in Animal, Corpus, or Herbam, they are still more effective than he is at any one of those Forms, because they need to raise a single Art, they gain greater benefit from an Affinity/Puissant Art, and they can leverage existing lab texts in a way the Elementalist cannot.
c) Elementalist is a really weak Virtue on its own!
I would probably allow a Greater Elementalist, through another Major breakthrough, to extend Vim into the Platonic Fifth Element, allowing both Mentem and Imaginem effects to be accessed as effects of that element with at least another elemental requisite.