Paradox releases a great deal of dlc for its grand strategy game. The quality of these dlc's varies wildly in quality and scope from overpriced cosmetic and music packs to dlc that completely changes the game.
Res Publica falls somewhere in the middle. It changes gameplay considerably for republics by overhauling their political systems, but unfortunately there aren't that many in any historical start and the ones that do exist are very small and weak.
RP offers additional features for the most important republic of the era: the United Provinces of the Netherlands. Even with the improvements Europa Universalis is not capable of simulating that republic's strenght and versatility correctly. Unlike the historical Netherlands they usually get overrun by more powerful neighbours.