Rhoynish remind me of the Gauls and Celts who fought the Romans, fierce but motley and disorganized. Had they stayed in Essos and after the Doom of Valyria, could they have stopped expansion of Volantis and the Slave Cities and defeat the Dothraki?

Probably not. Keep in mind, Nymeria’s ships were filled with desperate refugees, the fighting strength of the Rhoynish had perished at Volon Therys, and the victorious Volantines and Valyrians were settling down. If they had stayed in Essos, they would have all been enslaved as Druselka and her followers were when they turned back to return to Mother Rhoyne. 

Given the scale of enslavement that followed the Rhoynish Wars, the majority of the people who live along the river today are likely Rhoynish in ethnicity, but the Volantines still rule. For now…