Scions are different from those "Marked By Power." Scions are literally the descendants of an entity of power with mortal blood. Changelings can be considered as Scions if you chose to lump them as such, but in theory, anything that is a mix of a non-mortal supernatural entity and a mortal can be considered a Scion.
Some non-changeling examples from the series include Mouse, Glau, and Kincaid. Unless you knew exactly what you were looking for, or viewing them via the Sight, you would have no idea that they were non-mundane entities. Therefore, not marked by power.
If you are familiar with mythology, all of those children and descendants of gods are technically Scions. Hercules and Perseus are probably the two easiest to name and remember in Western cultures, and if you remember them, they appeared just like normal people, except they were "better" in some ways.
Now, I realize some of these things could be compelled by the GM, but I feel that having the blood of something ancient within or its mark upon you, often warrants more than just an aspect on your part. Maybe you are a love-child of some almighty dragon, but you might take after your mom, or perhaps you were spawned from the metaphorical loins of Venus, but perhaps your powers lean towards sexual fertility instead of love/lust.The "balancing factor" should be the character's high concept. Your Decedent of a Water Djinn shouldn't be throwing blasts of fire around, Ferrovax's Favorite doesn't run around with dragon's bane, Coyote's Touched probably isn't overly serious and blunt, A Dryad's Daughter probably can't arm wrestle a troll successfully, etc.
In short (otherwise I'll keep on rambling): I feel that 'Marked by Power' is something of a balancing factor, so you won't use your bloodline just to justify loads of powers. I am in no mean experienced with this game or system, so I am just spewing my thoughts on this matter :)
Having said my piece, I was also thinking about types of Scions myself and I had to think about the spirits often seen in Native American and Western-African mythology. Think about the Coyote, Raven, or Anansi (spider), Papa Legba. My thoughts about these spirits that they are not represented as a single, unified being, such as Odin, or Zeus, but rather a collection of spirits with their aspect. For example, Coyote is a trickster, but in the stories, he varies from a cultural hero, to a darker prankster, to the creator, to a teacher through trickery and as such there would be many spirits of Coyote, representing his different face. (As opposed to, for example, Zeus, who is a pretty defined type of deity.) They might all be linked to some communal well of knowledge or conscious, but not always and not actively.Sounds like a fun concept!
A Scion of such a spirit would most likely be 'Marked by Power', because it seems more legit to me that a spirit like that develops a passing interest in a human being, imparts some measure of power upon the mortal and then departs, only to check in every now and then. This means the powers of such a Scion could range from Sponsored Magic, to a non-Faerie variant of Glamour, or some shapeshifting powers. I would deem such spirits as too chaotic and whimsical to actually mate with a human being often, so they simply leave their imprint upon someone who impresses them sufficiently. This type of Scion could be relatively free the involvement of his/her 'parent', but, as befitting the trickster, he/she could be whisked away at any moment as well.
Marked By Power would certainly make sense for many scions.
But not all, and using it as a balancing factor seems unwise.
Partly because it infringes on the metaphorical space of an Emissary Of Power, partly because fluff should not be used to balance out mechanics, and partly because taking whatever powers you want is not something that needs to be punished.
You should definitely read the Custom Power threads. But not because they have anything to do with this, because they don't.
PS: What thread inspired this one? My guess is: http://www.jimbutcheronline.com/bb/index.php/topic,18620.0.html (http://www.jimbutcheronline.com/bb/index.php/topic,18620.0.html)
I'm just going to start by saying that if Sanctaphrax is right about the thread that inspired this one, then I'm just proud that it is still being read :-D
Now, for thoughts!
2) Interesting idea with the Totemic-style entities; I'm glad to see them brought up from time to time, and those times don't come often enough. While I like your approach with the "Marked by Power," I'm still uncertain if you'd really want to have it there, especially with a totem such as Coyote. As you noted, he's a trickster that does serve his purposes (good and bad), and if you look into the stories, there is always a lesson to learn from Coyote that may not have been obvious. In this example, why would Coyote want to leave something as blatant as a mark to be seen by everyone? How would this "Mark" prove to be beneficial? If it can be more of a problem, then why not leave it as the Aspect to be compelled?
3) Random thought: have you ever played the game Scion: Hero/Demigod/God by White Wolf? While the system is clunky, they do have some interesting approaches to these children with "divine ichor," whether in the form of magic or the things they carry.