I have been DYING to see Sims add dragons!!! An elf expansion would be the perfect place! There could be growth stages too!
Hatchling- about the size of a cat, can chill on your shoulders, maybe it drops scales like fairy dust that can be used for something? Cute regardless
Baby/Adolescent dragon - size of a large dog, still fits indoors, can probably be used to gather metals/gems
Adult - size of a horse, no longer fits indoors without massive doors, rideable, can possibly start fires
Bonus points if younger dragons can use pet doors because that would be hilarious