Mythic Blood goes back to the fine old ArM tradition of "Character Concept made Virtue." A Virtue (or Flaw) that stacks several Virtues and Flaws, with a net cost that seems reasonable. It happened a lot in 4th ed (especially the way-overpowered Faeries book).

Munchkin-ing aside, I have already ruled that the Mythic Blood Virtue also comes with the flaw "Storyguides Plot Monkey," and they can expect to have all sorts of things that inconvenience them based around a heinous plot of mine.

I've also said "No more than one in the group, I don't care what the blurb says."

Ex Misc does the same thing, with it's stacked Virtues. bleh.

So far as the Fast-Casting goes, and this also applies for your other posted question, I would suggest only allowing one Fast-Cast and one Normal Cast Spell per round, no matter the initiative. While there are no rules for how long it takes to cast a spell, the explanation can be that there is only so much power a Magus can channel. And no matter what the Munchkins say, there is no mechanic to increase that amount. Human Frailty and all that.

Still working on how long a Combat Round might be, as well as another poster's question on how far you can move in a Round. The rather nebulous nature of 5th ed Combat is vaguely an improvement over the rigid "Magic Goes Last" phases of 4th ed, but only vaguely.

Just some suggestions, let me know if you come up with some other answers.
J.

Your ideas about limiting MB, or at least making the most of the potential story aspects of it, certanly find resonanse in my ears. However your "Plot Monkey" solution, allthough it sort of follows the rationale for ArM5 Story Flaws, seems to me to ensure that the story will evolve around the MB character, thus further letting thar character dominate the story. Potentionally a rather unwanted consequence.

//I've also said "No more than one in the group, I don't care what the blurb says."//

Hmmm, could be good, lends itself to the players bargining for who shall be allowed to have MB character, thus adding to the cost of the virtue, in that you have to somehow "pay off" your fellow players.

//Ex Misc does the same thing, with it's stacked Virtues. bleh.//

I kind of liked some of the virtue packages which implied some sort of profession or function that the character filled in sosciety. But I wouldn't give more than a 1 point discount on the stacked v&f's. I haven't thought of ArM5 ex Misc that way, they do get a good deal don't they?

//I would suggest only allowing one Fast-Cast and one Normal Cast Spell per round, no matter the initiative. //

The question was tied to the wording of how fast and how much effort it takes to invoke the MB feat.
Independantly of this it is quite possible to fast cast more than one spell per. round. So if you would limit this, that would be a hole new can of worms so to speak.
But I was wondering if MB feat was actually meant to work exactly like fast casting, or if it was described like that just to give an indication of how quikly you invoke it.

Considering that any magus will only have a single Hermetic Major virtue, MB is allready an expensive virtue. I'd far rather have flawless magic, secondary insight or another of those that actually adds to your magical abilities.

MB is useful for a beginning magus, but not as much for someone with a decade or more of experience.

(Remember that with flawless magic, you can easily have ALL your formulatic spells fastcast)

//Considering that any magus will only have a single Hermetic Major virtue, MB is allready an expensive virtue. I'd far rather have flawless magic, secondary insight or another of those that actually adds to your magical abilities.//
I'm not saying that there aren't other more useful virtues, especially if you play a long running saga.
I'm mainly wondering if the MB feat is to be considered a fast casting spell - potentially usable several times per round, and in combination with other fast cast spells, OR if it's only described that way to give an indication of speed and ease of use, and that ist's usable only once per round?

//(Remember that with flawless magic, you can easily have ALL your formulatic spells fastcast)//
Se my other thread about problems with fast casting.
Flawless magic is high on my list for Major Hermetic Virtues too, but it's only worth considering if you know you want to master a lot of (,all your) spells.