Long time ago, I read a Doctor Strange novel where he was facing himself from another dimension (like in the movie), but this Doctor Strange was more powerful than him because he granted himself a few extra pair of arms, making his incantation more powerful.
I like the weird logic: no gesture = casting penalty, more gesture with more hands = more power.
It lead me to create the Incantors of Chimera. These mystagogues seek to empower themselves to become more powerful casters, possibly eyeing to become magical beings (I have to work more on the background and origins).
Some of the virtues are:
Vulgar alchemy and Inscription of the Soul
Potent magic (minor/major)
Spiritual familiar
I am thinking of two new virtues: Perfection of the Chimeric body (Hermetic, minor or major ?): the magi is able to invent MuCo spell that can improve his casting or magical abilities. Non-initiate can invent similar spells because they use existing guidelines, but they don't get the magical bonus.
So far, I am thinking of two possible spells.
The Skilled Incantor: grants an additional pair of arms, adding +3 to casting total, +1 botch die. MuCo 20 (Base 5, +1 Part, +2 Sun). Higher level spell could grant more pairs, increasing by +3 the casting total each time (or not if too unbalancing).
The Blessing of Zeus Triophtalmos. Grants a third eye on the forehead. As long as this spell is effective, all perception-based spells (detection spells & such) lasts for the duration of the Blessing. It basically grants the effect of Maintaining the Demanding spell with a MuCo (and a specific virtue) for perception based spell.
First question, should this virtue be Minor or Major ?
The Second virtue I am thinking of would be: "Channeling the Inner Strength"
The magi is able to enchant a ReVi into his familiar bond, allowing him to borrow a power or maybe even a virtue from his familiar. While doing so, the familiar is immaterial, and moves into the magi's body. The familiar cannot do anything while in this status. (I have not figure out the level for that enchantment). One enchantment is required per power or virtue to be borrowed.
Second question, should this virtue be Minor or Major ?
I worry that The Skilled Incantor could scale up dangerously, and would suggest no more than the +3. (perhaps "The more arms you add, the more difficult it is to control them all, resulting in a net zero) unless a higher-mag spell is invented with a Mentem component to allow for greater cognition - that could also scale alike to the "Increase a Mental Characteristic" guidelines for balance.
Alternatively, you could make it such that the +3 applies to rolls related to making multiple castings of concentration spells. (with one pair of hands doing gestures to maintain the first spell as the second pair of hands effects the second.) So while you're not raw-force more powerful, you're able to juggle more things simultaneously. It might make for a more interesting playstyle.
All this said, I do think it's a fun idea. Have you looked at Hindu mythology/symbolism at all, with their many-armed gods? Might be some fun inspiration there.
I refrain to get inspiration from the Indu pantheon as it won't be jelling well with Mythic Europe. At most, they would be handled as Faeries, similarly to Egyptian Gods and I don't see how faerie could be used as basis for an hermetic mystery cult.
But yes, between Vishnu and all the other deities, it would be a wonderful source of inspiration. But I am sure if I dig a little bit into European mythologies, I can get some interesting ideas. I might look at multi-headed, allowing bonus for fast-casting/multi-casting...
Regarding the potential for brokenness with multiple arms, yes, infinite scaling would be bad, so maybe limiting to two versions a +3 and a +5 - after that, you got eight arms that makes you enter the domain of the Spider...
Major, although you may want to account for an automatic minor or major flaw when the chimeric feature is shown. Having a bizarre third eye or four arms and hands is going to automatically alarm mortals. Magi will probably find it unsettling.
There's also six-armed giants adapted from Greek myths as the Gegenees in Mythic Locations (p29). They get three attacks, which they may split between different targets; it's not written up as a Power or Virtue, though which would have been my preference.
They also have the flaw of Monstrous Appearance which seems appropriate for someone walking around with multiple arms, eyes, or other things.
If there are no side effects, I would just use ReVi Maintaining the Demanding spell and build it from there: ReVi 25 (Base 15 (effect up to level 25), +1 Touch, +1 Conc), +5 maintain conc, +2 (3 uses/day) for a final level 32.
By using Conc + maintain Conc, it allows you to stop the effect whenever you want. Adjust the level and number of use according to the number of points you want the power to be.
DE makes a significant change for maintening the demanding spell base guideline: you cannot maintain a spell longer than what you would have been able to concentrate on it. So 15 minutes per concentration score point. Maybe include in that "major virtue" (which could be read as 3 minor ones) a minor one including puissant concentration besides a double (minor) lesser power of demanding spell may be a good idea thus.
Definitely want to put down a firm limit on scaling bonuses. My power-gamer translation of your proposed virtue would read "You get bonuses to your magical abilities with any self-transformations if you can make a good argument for it." The 'extra arms' spell is already a strong one for martial characters, and getting casting bonuses makes it better and better. A single +3 to all spellcasting is WAY to strong for a single spell. I would probably make it just remove penalties for subtle gestures due to having more to gesture with.... but yeah, importing cool ideas from other sources is really tempting.
If your goal is to have people changing their bodies to make them better spellcasters, I'd suggest more like this:
Perfection of the Chimeric Body (major)
This virtue allows the magi to enchant permanent physical changes into their bodies with reduced penalties and increased capabilities. Specific physical changes, such as extra eyes or limbs, tougher skin, or larger or smaller size, if enchanted as continuous enchantments on the Magus, do not cause additional warping for long term magical effects; They have become a part of their Essential Nature. In addition, these extra body parts can be enchanted to increase magical effects, and metamagic enchanted into the Magi's chimeric body can be used on the spells they cast. The mechanics of this requires the Inscription of the Soul to be useful.
This is also more extreme of an ability than I would allow in most of my own sagas, but also doesn't just give the caster a steadily incresing casting bonus based on how good his Mastery of Extra Arms with Fast Cast and Multicast is.