Currently existing mercenary clans and cultures mentioned in lore only (Palaic) already have plenty of potential to be used as destabilizing agents in Calradia. We have nord mercenaries, disgruntled former legionaries, separatist rebels, desert nomads and all they do is jump around boosting factions. Instead of bringing some novel faction from overseas, why not include a mechanic for minor clans to increase their overall presence/power in Calradia?
Take the Legion of the Betrayed and the Brotherhood of the Woods, for example, both of those clans originate from a group of people feeling betrayed by their overlords, one composed by former professional soldiers and the other by rebel peasants, both of them could have a set of goals to increase their power/presence/size, which the player can help, hinder or completely ignore, that could lead to a full rebellion, something like what we had with claimants in Warband.
I would much rather be involved in maintaining the status quo or helping minor clans become powerhouses than be stuck fighting armies that appear out of nowhere for no real reason.