@Goldfish, Spell Sieve is a trap option. A great many spells are very useful against players, but situationally terrible against enemies. I think that Spell Sieve looks nice, but is actually subpar compared to other, better choices (like Touch spells at range !).
And if you're taking Inspired Spell, you may as well take Wild Arcana with it too. Trying to compromise here doesn't make much sense. Unless you compromise by making Wild Arcana a swift action, but by limiting it to spells known. That would be nice, but would fail the "no homebrew" objective.
Of course, I think that suddenly making Lya a mighty spontaneous spellcaster doesn't make much sense, but what have you.
People, I'm resigned to the OP-ness of Mythic. I'm not on some crusade to "gimp" our strongest PC (yes, Lya's Capstone is definitely stronger than Visery's, especially if she crafts her own sweet artefact gear in the near future). No, I just feel a little uncomfortable with pushing Lya into a full crafter role (some proposals stank of that, making me nervous about the future) or making her a sorcerer with all spells known.
Also this is inaccurate, as the tier just confers the Longevity Mythic Ability, and it's conditional Longevity at that, making it arguably less powerful. The part of our Path, which all Mythic Heroes get by default, Immortality, just makes your death conditional based on someone having the right McGuffin to do the job. No shattering of world-spanning nations necessary.
Again, as has been pointed out, arguments like these make your suggestion of coherent theming ring as considerably secondary in your concerns for building, which makes anyone who really likes mechanical gains leery of anything you might propose.
I can't believe I hadn't realized that!
Well then, here's another nitpick about Snowfire's stuff: We get to bestow cleric spells of up to the 9th circle without taking the relevant Mythic Path Ability 3 times? Damn, that's a nice upgrade. I guess that having to wait for our Mythic Rank to rise is a downgrade in the short-term, but an advantage in the long-term?
Ah well, it's mostly fluff anyway.
Still, overall I am very happy with Viserys' Ascendant path. I'm just nitpicking here. Please tell me if I've misunderstood how that works too.