The only issue is that petty kings were handled automatically by ck2 by a cultural basis. So Þjóðkonungr would show up as the petty name.Pretty much. 'Þjóð' in Old Norse seems to denote a nation, like the Norðmenn (Norwegians), Danir (Danes) or Svíar (Swedes).
'Þjóðkonungr', as it's used in Heimskringla, is describing Gormr and Eiríkr as kings over all Denmark and Sweden, respectively; whereas Haraldr Fairhair only rules Vestfold in south-east Norway at the time he proposes to Gyða. She's basically mocking Haraldr's petty kingdom in comparison to the unified, national kingdoms of Gormr and Eiríkr.
Also, in the Viking Age, most independent rulers would have been titled 'konungr', regardless if they are kingdom-tier, duchy-tier or county-tier size in Crusader Kings. So in 867 AD, all of the independent county-tier rulers in Norway would be 'konungr'. As soon as they become vassalised, they would become 'jarl' or 'hǫfðingi'.
The major change in terminology of Old Norse landed titles occurs in the late 13th century due to heavy influence from the Holy Roman Empire, with influences and borrowings from Middle Low German into Old Norse vocabulary. But for both the 867 AD and 1066 AD start dates, 'feudal' titles are still quite foreign in Scandinavia.
It would be better to have konungr stay as king and have Þjóðkonungr be a conditional title for the 3 nations
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