Maxperson
Morkus from Orkus
I think you are probably correct if looking at ancestral ties. Dragonborn in the Realms are tied to a country that has not been written up as a location in 5e, and there's not a ton online, either. The DM would have to do some work to make those ties, buuuuuuut, it's also not likely to come up much in game play, at least not any more so than Thay would.I agree that Dragonborn would make great Flaming Fists, but my point was more on the story telling potential being a Flaming Fist has compared to something like a far traveler or from a remote poor village. I believe the position is Humans of Thay, the Dwarves of the Mithral Hall or the Elves of Highwood have far greater story potential for a PC than Dragonborn because they have deeper connections to the setting beyond a wandering mercenary.
Although if the DM makes it so that the Dragonborn species have a decently interesting connection/history to the setting similar to other cultures/groups/factions so Dragonborn don't feel tacked on then the whole discussion is moot. I imagine most here would do so, assuming your setting allows for Dragonborn of course.
Most of the storytelling potential for Thay and Tymanther would come up if you went to Thay or Tymanther. Otherwise you just have a bit on Thayan Enclaves and Red Wizards, and a bit about Dragonborn being mercenaries, fighting for their freedom in Tymanther, and being big on honor and tradition. To really dig deeply into either location would involve the DM going outside of 5e and/or making up a lot of stuff.
Mithral Hall and the High Forest(Elves of Highwood) are both along the Sword Coast/North and so are much more likely to come up in game play since those are the areas WotC has stressed for 5e/5.5e. They have the highest storytelling potential for adventuring on the sword coast.
Now if you tied your Dragonborn to the Flaming Fists and/or some other Sword Coast entities, the storytelling potential would also be fairly high.