Think as the dragon might. Whatever method he's using to alter his form, the dragon most likely is able to choose the particular details of his appearance, so consider what his initial purpose is in adopting the disguise. Form always follows function. If he's simply traveling and wishes to observe but not directly interact with people, then he'd likely assume an innocuous though mobile form such as that of a sparrow. If he wishes to simply converse with the party, perhaps to learn the local news or to find out what their purpose is, but isn't in the mood to overwhelm them by revealing his true nature, then he'd probably take the form a kindly old pilgrim begging to share a meal with them (if only to assess their kindness). If the dragon is seeking social interaction or an evening of entertainment, then it might assume the persona of a traveling merchant or minor nobleman with enough gold to buy drinks, tip barmaids and pay for minstrels. If he's on a righteous mission and, for some reason, needs to interact with locals he'll probably appear as a stalwart cleric of a righteous faith or perhaps an errant knight in shinning armour.
If, on the other hand, he wants to speaks with the party and wishes to impress them with his draconic awesomeness (but would rather not have to deal with the awkwardness of them peeing themselves) then he might adopt the cliched form of a tall, broad shouldered man in regal robes, cascading golden hair, a metallic sheen to his skin, glowing eyes, elongated fingernails, smoke curling from his nostrils and enough wealth displayed to make a king jealous. In short, he'll appear in whatever form will net him the desired reaction from those he approaches.
I suspect however that such an old timeless soul as a great wyrm gold dragon wouldn't feel the need to be so ostentatious. Why? He has nothing to fear, no need to impress and nothing to prove; he knows how awesome he is. I'd suggest having your dragon appear as a kindly old monk with a buddha-like serenity about him. He'd be dignified though unassuming, wise though humble, kind though detached. Or perhaps it could adopt the form of a small child; wise beyond its years though remaining light-hearted and innocent.
Hope that helps.
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