Well, the rules give general guidelines: you can make veils that are large enough to cover a small group of tightly space people (I'd say the limit might be around the size of a small car, perhaps), or you can change the appearance of something, using Discipline or Deceit to counter attempts to see through your Glamour in either case. Greater Glamours is more powerful in two respects: you get a +2 on your Discipline/Deceit rolls to pierce your illusion, and you can also create objects or creatures from ectoplasm -- which basically means you can create a freestanding illusion that acts as a real object or creature (ie, a 'illusionary' lion that could actually rip someone's head off!)
These abilities look as though they can be powerful if used correctly, though they are fairly limited in scope (for example, Greater Glamour can only create one thing at a time). I could see some extremely nasty possibilities for such powers used to potentially destroy characters, especially Wizards. Consider, for example, the Wizard getting attacked by a pack of ghouls. He proceeds to blast them ... then the illusion drops, revealing a group of very charred and very human corpses behind. I don't recommend this, though, as it would be a terribly unfair way to give a player Lawbreaker Stunts at best, and at worst cause the loss of the character to NPChood (zero refresh) or a Warden-sponsored weight loss program.
I'm not very familiar with Marvel's version of Loki, but I gather that his primary ability is to alter reality, allowing him to rewrite history and such. This is likely far too powerful for a player, certainly, and likely to powerful for an NPC villain, as well. Of course, a Scion would be considerably less powerful. Even so, I think that if I were making such a character, I'd go back to the core powers attributed to Loki in Norse mythology. He was a master shapechanger, a mythic deceiver, and had the physical powers of a godling. Depending on the power level you had in mind, I'd probably start with True Shapeshifting [-4], add in Modular Abilities [-7ish] (defaulting to Inhuman Strength, Speed, Toughness, and Recover, with an appropriate Catch), and put Deceit at the top of his skill list.
He'd then use his Shapeshifting abilities to mess with the players' heads. He could appear as the players' friends or allies asking for help, but the players learn later they were helping the Scion instead. He could get players or their friends in trouble with the mortal police, or even the Wardens. He could start making the players question whether or not everyone the meet is who they appear to be. Draw this out over a long period of time until the players realize they are being messed with by someone more than the obvious villains, then let them work on tracking down who or what is causing their problems.
