The Dragonborn are a new PC race in 4e D&D.
There was a quasi-race called the Dragonborn in 3.5 (introduced in Races of the Dragon). These were members of other races who were called upon by Bahamut and answered his call, becoming transformed into draconic creatures who hunted evil dragons.
The 4e Dragonborn aren't like this.
Instead, the new Dragonborn are a distinct race of honorable mercenaries who look kinda like humanoid dragons (and have breath weapons)
I'm not sure of the whole "race of mercenaries" thing. It seems weird to me. Moreover, the "honorable mercenary" thing is a bit odd too. I worry that they fill that inevitable Klingon spot in the race spectrum.
I don't really get excited about that sort of flavor. That said, 4e Dragonborn are mechanically sound enough. Giving them a bit of a different background might be interesting.
So... I'll look back at the 3.5 Dragonborn and their cousin the Mojh (from Monte Cook's Arcana Unearthed) for inspiration.
What do I want out of Dragonborn? I'd like some mystery to them. I'd like them to eventually (gradually?) become more dragon-like.
So, how about this:
The Dragonborn are not a true race: not as man and elf are. The first dragonborn were once a member of another race - and a person of some significance as well. The ritual to become a dragonborn is not a simple one, and it cannot be conducted on behalf of another. The ritual erases the new-dragonborn's former identity. Others remember her only vaguely, and her own memories are lost, returning only in dreams.
Few have the strength of will to undertake such a transformation. Fewer still have a compelling reason to do so... yet it is still done. Some people know that they were not meant to be as they are, and that their destiny is as one of the great dragons.
The dragonborn have a new epic destiny open to them: the Dragon. This involves a gradual transformation from dragonborn to true dragon.
I haven't decided if this sort of dragonborn breeds true or not. I like the idea of someone thinking he was raised a dragonborn, but then discovering that he was once a powerful human wizard or something who gave up his power for a second chance at life...