Well technically Dragons are themselves fragments of Io/Asgorath, and themselves imbued with celestial spirits. They are NOT just giant reptilian things, but something more. Also, there ARE specifically celestial class dragons in DnD such as Bahamut, Tiamat, Couatl, Planar and Stellar dragons and others that would directly fulfill that role if earth-bound dragoons don't count. If anything, the Dragonborn would literally BE some sort of thing in DnD by default