According to MRB, InVi spells require a different spell for detecting auras/magics of each realm (Magic, Faerie, Infernal, Divine). Could I instead simply add a magnitude for each additional realm, as if I was adding a requisite?
Ie, Base 2 - Determine power of a magic aura.
+1 Touch,
+1 also determine infernal,
+1 also determine Faerie,
+1 also determine Divine = InVi 10.
Or would that require a research breakthrough?
Also, could I then display that result in some kind of secondary effect? For instance, in one of the PbPs here, I have created this spell:
"The Significant Lamp"
InVi, Base 2: Determine Power of a Mystic Aura,
+1 Touch, +2 Ring, +2 Creo Ignem requisite = In(Cr)Vi(Ig) 15
Produces a flame whose shape and height indicate the strength of the magic aura within the circle. This spell is designed to allow magi to easily monitor changes in the strength of an aura.
Given the discomfort with portable ring-spells, we've kept the use of this spell stationary. But a portable device, item or talisman which displays the aura and other information seems very in-genre. Triggers based on a change in the aura are actually in the rulebook, but this is one of the few effects that I can think of which would take advantage of that.
As written, you can't.
I'd call it a fairly minor breakthrough though, if it only works with InVi.
It's a bit more problematic if it works in general, ie. Demon's Eternal Oblivion stripping might off of anything.
However, as long as you want the effect in a device, you can simply enchant it with multiple effects (4 effects of level 2? easier to install than 1 of level 10)
And then have a single Ignem effect triggered off any of them - you could even add a muto effect to make the flame burn wth a different color, depending on the Aura.
In the end its your SG's call. Adding extra mags for each aura seems fine, but not without adjusting for complexity (+1-2 mags) as well as the additional auras. This is because the spell should typically be crafted as four separate spells (akin to Mirror of Opposition: Form spells could be crafted to affect multiple forms in the same manner), but the magic itself needs to detect and switch in real time based on the environment. I think that is harder than just adding a new aura.
Otherwise craft 4 spells. They would be such low level (level 5 each) that you might be able to do all of them in one season anyway. Not ideal, but not a show stopper.
What would the spell do with places that have multiple auras? Perhaps only detect the active aura. There is also always the problem of affecting the Divine and the Infernal when they might not want to be detected.
I'd be fine with Ring for that duration. It is just as viable as Conc to me.
A magic item could even work around the Ring problem using D: Conc, with a Device Maintains Conc adjustment, and a few uses. It would be a great item to craft. Tellus is right about stacking enchantments with linked effect; neat solution, but not cheap when you need to add multiple spells.
If the magus can craft it with some increased mags, why not?
It seems plausible to craft a spell that shrinks and grows a target. Or Perdo Corpus and Animal as a wound level. The Magus still needs to understand the target creature during casting.
A jack of all trades DEO is not that dangerous. After all he gets decreased penetration because of the extra magnitudes of the spell. My troupe would see it as a BAD option, not an abuse. In fact we only have 2 realms (divine + infernal and faerie + magic) and that "meshing" of the spells has never been a problem. Magi can detect the aura by casting ANY spell anyway (like a level 1 whistle spell), since they notice if the aura is attuned with their magic or not. Detecting the exact aura shouldn't be that difficult
I said that your fist spell, the spell InVi with Requisites could work without problem, since to sense the affiliation could be granted thanks to others spells.