The game is designed to use multiple mods. I quote here a couple of the most popular, active also in previous editions of the games in this series. I believe that the most important mod for this game is No More Boxed Cars mod. It converts the gray rectangles we see on the streets at a quite large distance to normal models of vehicles. Otherwise, all the streets look like a big store of cardboard boxes. Other mods usually focus on adding a lot of new elements. These are buildings, decorations, roads, bridges, and many other attractions to make your game and the view of the city more pleasing but they don't have much impact on the course of the game.