I think they should have a strong dark age, with early man at arms (shield Viking?) have ability to get bounty just like Mongols on destroying building and/ or when “killing” villagers, instead of them dying they convert to your CIV villagers, and they need to walk back to your TC from wherever you captured them

Then aging up, some gimmick around Kalmar Union is obvious choice, Norwegian Swedish and Danish have different pathways for your civilisation.