I should have looked in on this Thread earlier - I did part of my Master's Thesis on Thracian units in Alexander the Great's army!
First, brief dissertation on What Is Known about Thracian military units:
They were famous for their 'light' infantry, originally called Peltasts (Pelta = Target, because it was a small round or crescent-shaped shield rather than a body-covering Hoplon). They were much less armored but much faster and more maneuverable over rough ground than Hoplites, but at the same time, armed with both throwing and thrusting spears, they could both skirmish and hold their own against enemy 'heavy' troops and cavalry. Thracians were a great source for mercenary troops, as well as Greek troops whose equipment was patterned after the Thracian Pelta/javelin combination. The Agrianes were an elite javelin-armed unit of Thracians that were part of Alexander's army from the time he became king until he died. Thracian nobles were cavalry, very few armored but all very good wth a thrown javelin from horseback. They were also, at least compared to Greeks, considered to be very good horsemen. They were also headhunters, as one Thracian cavalry unit tore up a Persian light cavalry unit and brought its leader's head back to Alexander as a trophy - Alexander was not particularly pleased, since he would have preferred the man alive to be questioned about what the Persians were up to at the time!
Later, the Peltasts became somewhat 'heavier' - the shield became a Theuros ('Door'), an oval shield almost as big as a Hoplon, and Thracian infantry fought in closer formations with a long thrusting spear and a sword as a side-arm. These were the 6000 or so Thracians who fought off 8000+ Bactrian cavalry on Alexander's right flank at Gaugamela, so by this time Thracian could mean either Peltast-javelinmen like Agrianes or regular Spearmen.
A Thracian Civilization is a little 'iffy'. Notoriously, the Thracians were never a single united people, but a collection of tribes who spent more time fighting each other than anyone else, and provided almost as many mercenary troops for other people (like Greek., Macedonian, Successor States and even as auxiliaries to Rome) as they did for themselves. The Greeks liked to say that if they ever united, the Thracians could rule the world, but they never did. A City State seems like a better 'fit' for Civ VI, and a City State that can provide 'mercenary' Peltasts to their Suzerain would fit their historical role.
A Thracian Peltast could be a Recon unit that has the factors of a Slinger (ranged factor, 1 tile range) AND gets a Spearman bonus against mounted troops. (The Agrianes, supposedly strictly light javelin men, butchered a bunch of Persian chariots at Gaugamela on flat, open ground)
A last note on Thracians, for a little Identification Confusion: Thracians wore tartan cloaks and used bagpipes (well, closer to a Small Pipe, but same principle): a Thracian Peltast unit could be a colorful little outfit!
Now about Mercenaries in general and Slingers in particular:
Slingers come in two types: professional slingers firing specially-made cast lead streamlined 'shot', and Everybody Else who herds sheep or cattle on foot, shooting rocks. The 'sources' for the lead-pellet slinger-mercenaries were actually two: Rhodes and the Baleric Islands, so Rhodes or Mallorca could either be a City State that gives you mercenary Slingers (factors and range like Archers, but +5 against Non-Gunpowder Ranged - the lead pellets flew so fast they could not be seen in flight, so you couldn't dodge them - they were deadly against unarmored archers or slingers).
Other potential Mercenary City States:
Okilis: gives you Celtiberian Infantry (Classical Era) (Swordsmen with a 1-Tile Ranged attack before melee - the Romans adopted both the long 'Spanish' sword and the heavy iron throwing Pilum from them)
Cirta: gives you Numidian Horseman (Classical Era) (Horse Archer who throws javelins, can move after attacking)
Berne: gives you Swiss Pikemen (Renaissance Era) (Pikeman with +1 movement, automatic Thrust Promotion)
Darmstadt: gives you Hessian Fusilier (Industrial Era) (factors as for Redcoat, if Garrison, adds +1 Production to a city)
In the next expansion they really need to turn the scout into a light infantry line.
Scout->Peltast->Skirmisher->Ranger->Commando->Spec Ops.
If I may suggest, a more complete 'Scout' line would be:
Scout (Ancient Era) - Peltast (Classical Era) - Forester (Medieval Era) - Dragoon (Renaissance Era) - Ranger (Industrial Era) - Armored Car (Modern Era) - Special Forces (Atomic Era)
- Or any part of that line that adds Something in the gaping hole between Scout and Ranger now!
