In order to make Wizard's Communion more useful and diminish its focus on increasing Penetration, I'm consideringing about the following house rule:
Instead of dividing the spell level by the number of magi, each extra magus makes it easier by 1 magnitude to cast the target spell.
If the spell being cast is a ritual, this also reduces the amount of vis needed (to a minimum of half the vis, plus 1 pawn). This does not apply if all the participants have the Mercurian Magic virtue.
Example: Three magi join together to cast a level 30 ritual spell. The magus who knows the spell must now compare his casting total against level 20 (i.e. 30 - 5 - 5). The spell only requires 4 pawns of vis instead of 6 pawns. The casting total ends up being 29, so Penetration of the spell is +9 and only a single Fatigue Level is lost (instead of 2).
Questions? Comments? Insults?
(And yes, I am also tweaking the Mercurian Magic so that it doesn't lose all its appeal.)
For some time we tried a House Rule with a similar goal. The idea was to make Wizard's Communion more similar to the Ceremony ability of many hedge traditions -- which is considerably more useful than Wizard's Communion. The House Rule was very simple:
The leader of the Communion must know the spell to be assisted by the Communion.
Every other magus makes a casting roll as if he knew the spell himself, and adds 1/2 the result to the leader's roll.
No helper may add a greater bonus than the level at which he knows the Communion.
This House Rule did, indeed, "feel" in play very much like Ceremony for hedge magicians. However, Ceremony gives a vast boost to the relatively weak abilities of hedge magicians. The modified Wizard's Communion gave a vast boost to the already powerful abilites of Hermetic magi. The result turned out to be unbalancingly powerful, and we soon abandoned it -- and toned down Ceremony instead!
One suggestion. If you want to try out a magic house rule in an existing saga, a good choice is to have it "enabled" by a one-off magical item/aura etc. In this way you can introduce it, and if necessary remove it, without a reality disconnect. If you decide to keep it, just assume some magus in the Order (maybe one of the PCs!) eventually manages to make a breakthrough based on the magical item/aura etc. and incorporates the discovery into Hermetic theory.
I note that by RAW each magnitude of the target spell adds one [possible] participant and requires 10 levels of Wizard Communion. I assume your version maintains these restrictions.
Wizard's Communion is a strange spell. Under the core assumptions, I think each extra participant will effectively grant something like +10 to the leader's roll, when all is said and done. This is too much; the 5-strong example communion of the core book provides a +40 bonus to the roll, whereas your version adds +20 which I think is much better.
I am not clear why you lower the spell's level instead of adding to the casting roll, however. I'm not sure there is much of a difference, but increasing the casting total means that there is no reduction of time for casting rituals (which seems sensible to me), and that it lends itself more readily to thinking of "powerful magic".
Similarly, I don't like the fact that your version of WC reduces raw vis costs. I think raw vis costs and time for ritual spells should remain as they are. If anything, the WC should make the rituals more complex and elaborate, not easier and simpler!
I am not sure whether a flat +5 is the best policy, or whether the amount a wizard adds should be proportional to his power. I am tempted to suggest that each extra participant add +1 per magnitude of the Wizard's Communion he successfully casts; thus a weak magus adds perhaps +2 or +3, having learned a level 10 or 15 WC, whereas a Rego Vim specialist might add +10 (a level 50 WC). This "feels" right, but might be exploitable. This also means that there is a point to learn WC at above level 10 [which by RAW is only useful to cover-up for having less magi than are allowed to commune together].
Mercurian Magic can reduce raw vis costs of rituals, and allow free Communion levels, as it does now. If you wish, it can add an extra +5, for a total of +10 - or alternatively +2 per magnitude of WC, under my suggested variant - so that Mercurian communions recreate the power-level of RAW ones.
Good point about the time. I agree that using WC should not make it quicker to perform rituals. I'll add a note on that. (OTOH, I don't think it should make it more complex and elaborate, either.)
The thing is, I don't want WC to be used only to gain higher penetration (which is its primary use right now, particularly for magi with MM). I want it to allow magi to be able to cast more powerful spells they wouldn't be able to cast otherwise, and more cheaply for rituals.
What I did with MM was give it the equivalent of a +10 bonus to learn/invent rituals spells, which brings back a reason for them to use WC to be able to cast their higher-level rituals. And the vis-reduction feature of my house-WC means that MM have some incentive to use WC for the casting of rituals with those without MM.
I see. In that case - yes, lowering a magnitude would probably make more sense than increasing one.
The RAW mechanics for WC work fine for that. The reason it is not used for this purpose is the spell-learning mechanics, that rarely make it an issue. You can consider adding a +10 bonus to all spell-learning, but that's a temporary stop-measure as the difference would still generally be low. Having plenty of Covenants-style laboratory improvements around will also help. You could also increase the effectiveness of lab texts, or simply allow unlimited spell-levels when learning from texts and/or from teachers. Finally and more simply - you could make heavy use of casting tablets.
Having magi join forces to cast a powerful spell makes sense to me. Having them join forces to cast it more cheaply... not so much. Your tastes obviously differ
Yes... I can see how the +10 works thematically, but I find it strange that they don't get the vis-reduction of WC. Why do Mercurian magi suck at WC? The canonic version has them excelling at WC, which is based on their traditions. I can see that thrown to the wind, but still... Anyways, mechanically it seems to work although the +10 is, again, rather limited and likely to pretty much drown amongst other bonuses, that will be more important to the spell invention and casting process. I also probably won't count it as a Major virtue, especially given its down-side, I think.
Actually, the ritual-learning "bonus" of my house rule is not a flat one. It's considering the learned spell as 2 magnitudes lower than it is. So a magus with MM who wants to learn Conjuring the Mystic Tower from a lab text needs a lab total of 35 instead of 56.
I see the WC as being invented by MM to get more magi to join in their rituals, and to allow MM getting together to cast their biggest rituals. The vis-reduction is just a "bonus" that allows MM to not be as heavily penalized when non-MM join in.
In RAW, I don't really see MM "excelling at WC". After all, they don't get anything more out of WC than anyone else, they are just systematically trained in it. And under RAW, I don't really see much reason for them to be (aside from penetration).