I'm not sure why you think it's reasonable for a non Gifted character to have Mythic Blood, it's made to be a virtue for characters who are Gifted.
If you're looking for a virtue that unlocks multiple powers to non Gifted characters, let me submit Heroic blood; And it doubles the value of flaws taken, allowing two virtue points for each flaw point taken.
Not what I was saying. I was repeating what you said back at you.
It's reasonable for someone who is Gifted, possesses Mythic Blood, is 40 years old, and has never been discovered or trained, to exist. And for that person, Mythic Blood as it is currently written just doesn't work.
Wouldn't a non-Gifted person with "Mythic Blood" just have it represented by noting it in his/her background and associating a couple of relevant virtues and a personality flaw with it?
As to a Gifted but non-trained possessor of the Virtue like your Shapeshifter with Animal Ken, use the free Focus as an Affinity with one of the Supernatural Abilities if and until they are initiated/have their Arts opened. If it's Affinity with Shapeshifting, then it could morph into a Minor Focus(self-transformation), maybe; Animal Ken could be fudged that little extra and give a focus in InAn.
That doesn't make any sense. You said non Mage, which to my thinking means someone not possessing the Gift. It seems perfectly reasonable to me that someone with Mythic Blood and who hasn't been trained (Hermetic or another tradition) could take advantage of all of what his heritage has to offer.
That same someone who is 40, Gifted might also have Flawless Magic, or some equivalent, that person doesn't benefit from the ability. There isn't a problem here. This virtue is for Gifted characters, that circumstance hasn't permitted they be trained limits how well they use their birthright seems perfectly reasonable. If his Arts were to be opens, it could be done by any Magis, rather than requiring an InVi specialis, as would be necessary with a Supernatural virtue.
A person who has the Gift isn't necessarily a mage. You aren't a magus/maga/mage unless you've got training.
And I don't think Flawless Magic is appropriate for a Gifted non-Mage, either. Like the core book says, Virtues provide a benefit to the character. Something that doesn't provide a benefit isn't a Virtue. It's kinda like the way a Flaw isn't a Flaw unless it makes your life harder/more interesting.
Sure, but Mythic Blood absolutely does provide a benefit to any character who takes it, regardless of their training, tradition, or lack thereof: The ability to automatically fast-cast an effect with (equivalent) Hermetic level + Penetration of 30, and to do so all day long with no die rolls, no fatigue, and no chance of failure or botching.
There may be other benefits that Hermetic magi gain which the untrained Mythic Blood character doesn't, but that's beside the point. The at-will spell is (or can be) a huge benefit on its own, even if the reduced fatigue from casting or the MMF don't apply to you. Indeed, I would even go so far as to say that the at-will spell is even more of a benefit for an untrained Gifted character than it would be got a Hermetic magus, given that the magus has access to spells anyhow, while the untrained guy would have no other way to access his Gift.
Unlike FM, which nDervish points out, you do receive a significant benefit from Mythic Blood even if you didn't belong to a tradition and couldn't unlock the full potential of the mythic blood. What is the true purpose in changing mythic blood, what are you wanting to do, exactly that mythic blood is preventing? It is likely we can get you there without changing mythic blood at all and instead picking an alternate virtue package. Nevertheless, it is conceivable that someone does have virtues that they can't access. In Apprentices, these are called Inherited Virtues, in the core Ars book, in the apprentices section of the Laboratory chapter it is presumed that, in game terms, the virtues are possessed by the Apprentice prior to training, which is perfectly reasonable for Mythic Blood. They might have both Mythic Blood and Affinity with Art, and they could use some of their Mythic Blood powers, they couldn't derive any benefit from Affinity with Art.
If you change Mythic Blood to a supernatural virtue, you now require a Hermetic magus to have a lab total of at least 60 in InVi to Open that individual's Arts. InV1 60 is a challenge to someone who isn't a specialist in Intellego or Vim. If we say that the parens is skilled in Magic Theory with 6, intelligent of 3, and an average aura of 3, it means that In+Vi+lab bonus +virtues needs to be 48. And this discounts the fact that having an on demand power, like casting a spell equal to 30th level Hermetic affect is like a Supernatural Ability.
I think perhaps this person is working straight from the core rules, and thus does not have access to books that detail the alternate virtues that would fit the concept.
Well, that's why I was suggesting that we could help and explaining how virtues are acquired in other places. If one looks at the core rules, only, then my point is only stronger, that changing Mythic Blood into a Supernatural virtue will make it more uncommon in the Order. Not sure that's what Sanctaphrax wants, either. Of course, one could also just incorporate the house rule that Ovarwa just mentioned in the simple but huge house rules thread, but that compounds the problem, IMO, rather than fixes what, IMO, isn't broken.
But it is unfair for me to say "pay another twenty bucks if you want to play the game!". And I do not think micro-managment of all the nuances of the setting are a concern.
If it works for your game, specifically the saga being played at the moment, then go ahead and call it a Supernatural Virtue. Drop the Focus and Stalwart casting and substitute two other Minor Virtues that reflect what you are trying to achieve. Save corrections and improvements in concept for the next saga if you ever decide to acquire RoP-Magic. But this is not mandatory to having a good time.
In the same vein, if someone is intending to make these purchases in the future, knowledge can help prioritize those purchases, and anticipate the effect of the changes. I didn't say pony up $20 bucks to figure it out, i said where stuff was at. He already indicated a willingness to purchase RoP:Magic, which suggests he might be interested in purchasing other books.
People can ignore all kinds of stuff and have a good time. Even I can do it. 
I'm not worried about cost. Books are cheap.
But adding more books won't fix the problem. Replacing Mythic Blood with other virtues won't fix the problem. Because I don't want some particular mechanical effect on non-mages. I want Mythic Blood. Specifically Mythic Blood. And I want it to be the Major Virtue that it is.
Partly for the flavour of "this pirate is descended from Poseidon", partly for use with potential apprentices, and partly for magi who really ought to be able to take another Major Hermetic Virtue.
Strong Faerie Blood works well as a Supernatural Virtue. I'd like Mythic Blood to be similar. Maybe with a special "doesn't impede Art-opening" clause, maybe not.
Then I have no advice to offer you. Apparently then, you are enticed simply by the name "Mythic Blood", not by the actual Virtue. Any of these other suggestions will work fine. Just file off the name "Magic Blood" and call it Mythic Blood.
The reason Mythic Blood is for magi only and works the way it does is because someone made it up. All this stuff is make believe. It is a habbit of Ars gamers, and I can be guilty sometimes, to treat all this as some pseudo-science. But it is not.
Use whatever titles for Virtues you wish and change their effect as desired. Whatever works for your saga.
That would, unfortunately, cause another problem. A character sheet mismatch between Joe Dragonson before apprenticeship, Joe Dragonson after apprenticeship, and Jim Dragonson who never gets apprenticed. Which obviously isn't the end of the world - rules aren't physics, as you said - but I dislike it and would like to do something about it.
And of course it wouldn't let magi with Mythic Blood take another Major Hermetic Virtue, which seems like a shame.
I think you have more advice to give me than you think, though. For example, you can tell me whether you think a Major Supernatural Virtue that gives Magic Resistance 15, a magical feat, a minor virtue, and a minor personality flaw is balanced. And if not, whether knocking the Magic Resistance down a bit would make it so.
What if I told you that you can accomplish all that with a 0 point virtue?
I'd be interested, and curious about what drawbacks that Virtue has. Because that's a lot of power to get for free.
My only input is that a Major Virtue giving you much other than MR is already pushing it. Maybe a little soak, maybe an occasional die roll boost (Guardian Angel, True Faith). Though even that comes with an "easy to lose" drawback. Seriously, significant MR is Major Virtue-worthy already. Major Flaw-worthy, too, considering you basically achieve KoS status with the entire Order if you aren't yourself a magus. If this happens pretty much any time a Magic being cross-breeds, well, for one thing the Order would react INCREDIBLY strongly to it and would be in many small ways warped by the constant drive of militant Order members to scour the landscape (physically and with Intellego) to keep tabs on as many potential magical children as possible to remove threats to the Order... And for another, the intensity of the Parma Magica's singularity as the non-Divine/Infernal MR would be significantly diminished.
I wouldn't mind a revision of Mythic Blood. It's a lot like Secondary Insight or Elemental Magic in terms of comparing itself to where Major Hermetic Virtues should be at. And I agree that there should be a way to portray things like Mythic Blood on a totally mundane character, Powers notwithstanding as you don't own RoP:M.
But if you want the character to have Magic Resistance, just give them Might. You can give them Might in the form of a Virtue, mind you, but just keep the mundane Mythic Blooders away from the Magic realm-associated MR. Half-dragon, half-human characters can be built quite easily with Might if you want that MR.
The virtue is Magic Human (from RoP:Magic) and it allows you to set a might level consistent with the power level of the saga, and the slot that the character would occupy (grog/companion/magus slot). With might you can choose qualities above and beyond the powers that Mythic Blood offers, and it satisfies your requirements.
Attempting to adjust virtues you see as being somehow wrong now, while there is an expectation that you will eventually acquire other books, risks making problems for you later on as you attempt to incorporate the things from the new book with the things that you've changed from other books. Beings with might tend to be unchanging. Faeries stay within the role, unless their role is over and then they pick a new one and are completely different, for all intents and purposes. Magical beings, according to RoP:Magic advance slowly, if at all, since the have drastic limits on how they learn. Learning requires high SQ, in excess of their might, and they only get the difference of SQ-Might, although consuming vis does improve the SQ by 2 for each pawn consumed.
By all means, adjust Mythic blood if you think it needs it, but be cautious about adjusting if you plan on including other books in the future. Changing it now is a bit like changing a jigsaw puzzle piece now because you can't find the piece you want and requires two changes.
Akriloth, Mythic Blood is really pretty powerful, the right feat can do a lot, it's way more powerful than Secondary Insight or Elemental Magic. Stack on top of that no fatigue loss (and the right build can take advantage of this) if CT is within 10 levels of the spell, and then a free focus? No, it doesn't need anything else added to it.
I didn't say "added to," I said "revised." Though I suppose I don't think especially highly of it in any case, but that's mostly because of personal experience... None of my players like it because it removes access to their favorite Virtue (Major Magical Focus) and Penetration is usually a big deal in my games so they tend not to even invent spells that might cause Fatigue... So eh. I'm probably seeing it from a skewed angle.
Anyway, I do agree with you on the Might thing.
Well, any focus that can be used with preferred Arts can achieve high penetration, whether major or minor. And then consider not being fatigued from all of the other magic that falls outside of the focus. Does a weather maga need high penetration for all of her weather spells? If she is mostly blowing wind around, is mMF: Wind enough, rather than having an MMF:Weather? It's true that the weather one is broader, but wind magi can still do a lot of damaging things with wind magic, albeit a bit indirectly.