This mastery was invented by Jana of House Tremere (Translyvanian Tribunal) in 1219 and quickly spread to members of House Jerbiton and other magi who has decided to set a personal flair to their spells.
What this spell mastery effect does is that for every level of spell Mastery you can have one special appearance of the spell. For example with Spell Mastery 3 on my incantation of lightning, I can have the standard Incantation of lightning, as well as one with green bolts of lightning, one that looks like a bird of lightnings flies straight towards the target as well as one that looks like a ball lightning rolling towards the target.
There are no mechanical effects in damage, range, target or duration. Just that the spells looks different from the standard way it looks.
I fear no one would want to take it. They could instead reinvent the spell from lab notes in a single season. They would then loose the advantage of other spell masteries for the spell but it seems that this might be a smaller deal tan taking another level of spell mastery.
There is the spell mastery that allows other spell masteries to be used with another level general spell, I think it could apply in this case as well.
I beg to differ. I think plenty of Mages would like some style with their "Signature Spell", and I would swear there are some Spell Mastery effects that get better the more ranks you have in Spell Mastery, so once you have made your "effective" choices, you might still want to raise your SP level in that spell. Why not some fluff?
Yup. It has more uses than pure aesthetics. I also agree that it might be a worthy feature for a spell. If your sigil is horse dung, the taste of herbs (MuIm5) spell might not be a very good spell on its own, for example.
Flourishes are described on page 60-61 of Houses of Hermes: Societates and basically allows a maga to subtly alter her casting sigil with a Finesse roll. The sample uses a character with a sigil of "oranges" which usually manifests as the smell of orange blossoms, but could be with a flourish to exhibit: the colour orange, the taste of oranges, or a citrus smell...
They don't require any special Mastery or virtues, but are generally exclusive to House Jerbiton anyway since the power has few practical applications (your sigil is still easily recognizable to Quaesitors) and thus only practiced by magi who care strongly about the aesthetic aspects of their magic.
I think that's the point. It's not meant for anything but an aesthetic display. It's not as if the mastery does anything useful, either. One could argue that some mastery abilities are better than others, but this one does nothing but changes the appearance of the spell.
In my opinion, that makes it the best mastery choice 8).
I love the idea of being able to bend the spell for aesthetic purpose. There could be other ways to implement this : virtue, competence ("fashionable magic" ?), but I like the idea that someone that has developped a special dedication to a particular spell may be emotionally bounded to that spell and want it to be more than just a tool.
besides, aesthetics may also be useful in some situations : would you want to cross a sodale that has thrown 23 [strike]trivial ignem spells[/strike] balls of abyssal flame without tiring at last year's tribunal contest ? Certainly not if you failed your PER rolls...
Personally, if I were running it, there wouldn't be any mechanical requirements. If the player wants to use his Mastery level to do something interesting, which allows him to have fun, and which does not risk unbalancing anything in a mechanical fashion, I'd happily just let him do it. Spend the XP on Mastery, take the new power, describe it however you wish.
Technically, if you wanted to make him invent it, it would almost certainly be a Minor Breakthrough. But who would want to spend a decade working on something like this?
Another alternative, if you really want to make sure your mechanical bases are covered, would be to say another magus of the Order has already invented it. Put the new Mastery ability in, for example, the Bonisagus codex which gets published every 7 years or so. Perhaps there's a tractatus which describes Spell Mastery: Aesthetics sitting in the ruins of an old covenant library somewhere. Now, the PC does not need to spend years inventing a breakthrough, you get a good story out of it, and you are not "hand-waving" anything at all. This is exactly how new aspects of Hermetic magic are disseminated.
I'm considering doing the same thing in my Saga, with a new Target: Pair. I don't see any of my players inventing something like that, but it's just so damn useful and makes so much sense, that I can't see why it hasn't been invented yet. So I'm going to introduce a tractatus written by some Bonisagus somewhere who has already done the work.
If one is inventing a spell mastery that provides some kind of game mechanic advantage, I would look to the Original Research rules, as a minor breakthrough.
This is more Art than science, though. For this specific instance, I don't think it requires a mastery at all. I suggested looking at Flourishes, which relies on using Finesse to make things look cool. Finally, there's the spell itself. Spells can be designed to look as if they have cool effects. Spells like The Incantation of Lightning, Ball of Abysmal Flame and Pilum of Fire all create their T:Individual of the Form at the (location of the) target (recipient) of the spell, but it appears to shoot from the hand of the caster.
 It's problematic if one creates Fire/Lightning at the exact person and suggests that a Wizard's Leap might not defend against such a spell, if the person were to Leap and still be within range of the spell.
Isn't this what multiple casting can do? I suspect that this would be between T:Ind and T:Group, and would only be a +1 magnitude, rather than Group's +2, but it would share the same requirement that Group has that the items be closely linked, somehow? And what happens when someone masters a T:Pair spell with multiple casting?