As the module says, you can pretty well place it anywhere you want. You have to consider, thought, that Thracia was a moderate to major-sized city-state between 1200 and 800 years ago; before that time it was fairly isolated, and thereafter it was a backwater.
So you need to take that into consideration, if you worry about timelines and such in your campaign. This means, for example, it can't be placed too closely to Viridistan or Tarantis.
The timeline does fit the Roglaras well; you could say that Thracia was a settlement of Viridians who, when the Empire collapsed in the east, rose to form their own kingdom. When Thracia declined and then finally fell, the City State of the Invincible Overlord arose to take its place.
Similarly, the timeline fits well with the collapse of the Orichalan Dragon Empire; you could say the Thracians were non-draconic Orichalans who built their own kingdom after the collapse of the Dragon Empire.
The best fit, I would say, would be somewhere in Barbarian Altanis, though that just might be my preference for placing the origins of Greek-inspired Wilderlands cultures on the Pazidan. Thracia would have grown between the falling empires of the Dragon Lords and Viridistan, close enough to affect the local region, but still a modern backwater. As the best location is in swampy hills near mountains with jungle-cloaked areas nearby, I would go with one of the following hexes in Barbarian Altanis (Old Map 2, New Map 8): 0825, 0826, 0926, 2410, 2411, 2511, 2715, 2716, 2923, 3022, 3119, or 3219.
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