I am trying to figure out how to create this mod myself, but the point is, I want to make a mod that brings back the original domination victory e.g. conquer all original capitals on the map. Any ideas on how to go about this?

I would say that because of how they changed capitals and how they can change per era, you would just have to capture the capital of THAT era. That could also make it spicy because if the Exploration Era ended before you win, in the Modern Era, players will have new capitals, so you'd have to restart. Might have to break some long alliances, even. Although I guess original capitals would be more authentic, so I can go either way with that. If it's original capital, there's got to be a way to visually tell which city that is - I guess the capital star, but grey or outlined.