This would be unbalancing for the rules and also reduce the value of inventing formulaïc spells. It could be good if your player are reasonnable enough to not abuse it.
True.
But not quite: you can allow Wizard Communion to help with Casting tablets.
For use with Wiz. Com. I would rather see a breakthrough that enable a great mage to invent a spell in many parts, so that young magus can learn them each part more easily. E.G.: An Aegis level 60 in three parts of level 20 each. That spell can only be cast using WC, each mage "casting" his own part.
But the inventer needs to be able to invent the level 60 spell, so no young magus can use that breakthrough to invent spells he could not otherwise - thus preserving game balance.
It's dependent upon the primary caster's knowledge and arts, but not anyone else's. A magus with deficient Creo and Ignem can still help cast a CrIg ritual with Wizard's Communion and that's daft. That, and I feel Wizard's Communion is too cheap - it has very few drawbacks.
Frankly, I'd prefer a CrVi spell which boosts the flow of magic, granting him a +1 effective aura bonus and have that be a non-Hermetic effect. Verditius could boost auras, after all, so it's reasonable in canon and worthy of research, and it's make groups of magi more powerful in terms of penetration but not obscenely so. (Note that I realise the rammifications of this are vast, but I just wanted to give an alternative to Wizard's Communion as an example).
Something I've missed from ArM4 was the existence of casting foci - items which when a spell was cast using them gave some small bonus to the casting total based on the Form and Materials bonuses. To a certain extent,
Potent Magic recreates this, but the spells then require the casting foci. I think Potent Magic should be divorced from this (since magical specialisation and the incorporation of items into spells don't really mesh perfectly).
As such, here's a proposed research item: Casting Foci(Minor). The magus invents a spell incorporating a casting focus (one and one only), such than when the spell is cast using said focus, the form/material bonus is applied to the casting total. The maximum bonus is limited by the magus' Magic Theory, and double the desired bonus is added to the spell's level for the purpose of inventing the spell and lab texts. The focus is not required to cast the spell, but no bonus is gained without it.
For example:
Pilum of Fire with a focus of a spear made from brass, using the full possible bonus of +3. The mininum magic theory required is 3, and the level of effect is 21 (15+2*3), requiring a lab total of 42 to invent in a single season.
Possibly as part of the same breakthrough, possibly a seperate one: Spontaneous Casting Foci (Minor): The magus may incorportate a a form/material bonus into a spontaneous spell, limited by his magic theory, gaining a bonus equal to the form bonus to his casting score. The spell must be cast using ceremonial casting, and the number of botch dice is increased by the total bonus sought. Thus a magus with magic theory of 7 gaining a +5 bonus to his casting score from using Red Coral in a spell against demons also gains 5 extra botch dice. He could have chosen to gain an additional +2 (taking it to +7, his magic theory) but at the cost of 2 additional botch dice.
I do prefer Potent Magic because of the trade-off between power and convenience. But since it is a breakthrough, that's fine with me. Might be a bit much for a minor one, though.
The Jerbiton chapter in Societates already provide that kind of things: a prepared location speeds up casting, props provide a bonus.
Ordinarily, I agree in that I like power vs flexibility trade-offs. It's just that in this case, it feels to me like something which always was and should be in the scope of normal Hermetic magic which was cut from the core for space reasons and then reintroduced as something special. If I had a single complaint about 5th Ed, it'd be the growing emphasis on mysteries cutting away at the core of Hermetic magic by explicitly making things the purview of mysteries.
True, but they're generic. Whilst I find the idea that prepared area can speed up ceremonial casting to be a good one, the props themselves are odd. The bonus has nothing to do with their value or mystical significance, just their size. It's an abstraction which I don't feel adds anything - it emphasises that magi have more power in their labs by adding power there specifically (where else would large props be kept) rather than by having it be where they work, well equipped and undisturbed.