This is what a small rural US town looks like in vanilla Cities Skylines. The colors are way too bright and yellowish, the assets don't look American and nothing else really fits the style we try to go for.

This is what our village looks like with a good map theme, in this case the Cedar Valley 4K map theme. I advise using the Environment Changer Mod to easily switch between map themes when loading a save game. With a map theme you set the general feeling of a map. There are arid and desert styles for cities like Dubai or Los Angeles, tundra styles for cities in Northern Europe or Canada, snow styles or more generic ones as well. So choose whatever you see fit for the city you want to build,
no matter if US based or not.
Attention though, map themes, especially 4K resolution ones, can decrease performance due to being multiple megabytes large. This means that they have to be loaded into RAM and VRAM. The performance impact generally however is very small.

A good LUT changes brightness, contrast and hue of colors and in this case makes our scene look more realistic. How about we go with the Photorealistic LUT by Reaper? Or you can choose a bleaker LUT for more northern cities. Or how about a bright LUT for desert cities? The LUT should somewhat match the previous chosen map theme, so keep that in mind. Oh and did I mention the best thing: LUTs have absolutely 0 fps impact! Now back to our scene: the whiter areas look too bright, but stay with me, we are changing this in the next step!

We are using two mods in this step to achieve the picture above. In order to achieve better lighting with less stark contrast and yellowish hue, we are going to use the Dailight Classic mod and combine it with the Softer Shadows mod. Both mods have no fps impact at all and this step applies to both US and other cities.

Let's increase the lanes on the main road to 4 and the residential roads should be a little smaller. To achieve this we use the Network Extensions mod. The problem now is that the roads don't really look american and lack the double yellow in the middle. Changing the road texture to a US-style is achieved through the Road United: Core mod and by downloading one of the US based texture packs. Together with the texture pack from the previous step the whole scene now already looks more realistic. The Roads United mod also is capable of switching to German, UK, Russian and other country roads as well, so feel free to choose whatever fits best for your city.
As with map themes, keep in mind that the textures can be quite large and take up some space in your RAM and VRAM. If you are low on both this can decrease fps. Otherwise the fps impact is neglegible.

We are now going to change the traffic lights to the American version. If you build a European city this is not needed.
But how about we take a llok at another detail that can greatly enhance the immersion of your city. With the mod Emergency Lights Changer Plus we can choose between blue only (European) or blue and red (American) or red only (Japanese) emergency lights for ambulances, police cars and fire trucks.

It is now time to download US style houses, shops and cars from the workshop. In order to easier assign the new buildings to a theme we are going to use the Building themes mod. There are hundreds of assets available so enjoy choosing the ones that you like! Attention though: some assets are way more graphically complex than vanilla buildings and might slow your PC down. Also you will need a lot of RAM because the game will need to load all the additional assets into it. So be mindful of how many new assets you download. Together with some custom trees the whole scene already looks way better than our initial vanilla screenshot.

This is the end result compared to the vanilla scene:

To make your game load WAY faster with many assets you will need the Loading Screen Mod!

US
Pre WW2 suburban houses
Suburban McMansions
Americana old houses
Brooklyn rowhouses
Ghetto houses (needs RICO mod!)
Manhatten growables
American 1900s highrises
Suburban Mall (needs RICO mod!)
1900s commercial
1900s commercial additions
parking lots (no US city would be complete without them!)
American vehicles

Europe
Berlin rowhouses
Modern European residential
UK terraced residential and commercial
Paris houses
European Suburbia official DLC (not a free mod!)[www.paradoxplaza.com]

Asia
Japanese suburban houses
Japanese commercial buildings