Varies a ton by type of dragon - I'd strongly suggest going with a lower-case-d dragon, though; they're still incredibly potent creatures, but a bit more approachable.
For example, a scion of an eastern style dragon might possess a Pearl of Wisdom that grants them flight & sponsored magic (with themes of water/healing/growth/storms & a sponsor-agenda of 'maintain balance & energy flow in the world') - and be otherwise entirely human.
Or such a character might have Channeling & Ritual: Water, Human Guise, Aquatic, Claws, and Inhuman Toughness & Recovery with a +3 catch, and use the traditional 'Pearl of Wisdom' as a focus item for their magic.
A western style dragon's scion would probably be more focused on physical prowess - and less capable of feats of personal shapeshifting. For such a character it might work well to add an Item of Power that grants Human Guise (and perhaps Glamours) - something he acquired by cutting a deal with a fae, or a gift from an otherwise-distant draconic parent, or even a device custom-crafted for him by the party's wizard (assuming there is one).
For an Emissary, agenda is important; start by putting the dragon in as a major local power, and see who its allies and enemies would be - an old and well-established dragon is almost always going to land on the 'preserve the status quo' side of that chart. An emissary of such a beast is likely to be tasked with whatever "minor" matters the dragon considers to be beneath its notice - which could range from negotiating with other local powers, to collecting tithes from the dragon's allies, to exterminating a nest of Black Court Vampires that's disrupting the feng-shui of the area by their presence.
Or you could go with the opposite spectrum, with a very young dragon that's chosen to empower a PC emissary because it saw something in that character it could use to secure its newly-claimed territory - perhaps the PC has a significant magical talent, or is the heir to some bloodline that allows them to wield a specific Item of Power that the dragon owns. (Side-note on this one: a young dragon is probably a vanishingly rare creature in the modern world, and likely to attract significant attention from powers looking to groom it as an ally for later - or to eliminate it now while it's still weak.)
Edit: Oh, and don't forget the unusual - a dragon might well include a quetzalcoatl style creature, especially if your game is set after Changes. I don't know enough of the right lore to be much help in figuring out powers for something like that, though.
