Oooo... I like it.
So let's see. First, who were the nephilim?
Genesis 6:4, from the NIV:
“The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of men and had children by them. They were the heroes of old, men of renown”.
Enouch 7:1,2 (canonical scripture included in the Bible)
“It happened after the sons of men had multiplied in those days, that daughters were born to them, elegant and beautiful. And when the angels, the sons of heaven, beheld them, they became enamored of them, saying to each other, Come, let us select for ourselves wives from the progeny of men, and let us beget children."
Jude 1:6
“And the angels who did not keep their positions of authority but abandoned their own home—these he has kept in darkness, bound with everlasting chains for judgment on the great Day”
Nephilim could be seen in a few ways.
A. They were powerful children of unions between human and angel (sons of God)
B. They may have been the 'titans' of greek folklore
C. They could have been the demigods of greek lore (Hercules, for instance) or the gods themselves
For that matter, following the Christian faith, there is no reason that the angels who 'abandoned their positions of authority' might not be the Olympians or the original titans. The 'titans' or nephilim could have turned on their parents, as the Olympians turned on the titans. Jude says that God holds the nephilim in chains. Perhaps it was from this that the tales of the Olympian gods turning on their parents came from.
Anyway, here is what I think we are looking at:
channeling(-2, -4 to use spellfire): soulfire/hellfire (which one? They were sired by angels who didn't do what they should. Hellfire then? Perhaps access to one or the other depending on the nephilim? I don't know. Can we just call it 'spellfire' or is that a copyright? I don't think it is.)
hulking size(-2): (remember that nephilim means 'giant')
Mythic recovery, speed, strength, toughness(-6 each):
True shapeshifting or human form (-4/-2)
That is about 32.
That is not counting things like refinements or, if you go the 'Olympian' route, different nephilim might have additional specialties. Poseidon, for instance, would be 'aquatic'. And yes, I put channeling soulfire/hellfire. I think of certain beings as more like their own sources of power. If you want, call it sponsored magic, but the being is their own sponsor.
And I really don't think you should have any wings in there.
So after that, I think anything from that list, but graded down, would be viable. The magic part, if included, should be something like this:
Evocation element: Spellfire
Add +4 to the refresh cost to use spellfire as an element. Spellfire is similar to soulfire, in that it comes from within and is far more broad in its use than regular evocation. Consider it to provide the full range of thaumaturgy spells, with evocation’s methods and speed, as mentioned in YS288. Since spellfire is not sponsored casting, you do not get the other similar effects, such as tagging an aspect for free.
Note: This is most applicable to beings of power 'far beyond that of mortal ken'. In other words, gods walking the earth, or powerful agents thereof. Many unstatted creatures in the books would have access to this power.