Currently one of the most popular mods on the Steam workshop, it introduces the West coast of North America to HOI4, including such canon factions as the NCR and Caesar's Legion, while also seamlessly integrating in its own lore about Mexico, Canada, Colorado, Oregon and other regions.
It has custom music, 3D models, an entirely different technology tree to the base game, a hand built map, an economy system and around 92 unique focus trees (not including all of the amazing submods that have been created). Currently the mod is in version 4.2 and will add in new content in the Northern areas of the mod in 5.0, slowly adding in the whole North American continent from West to East.
The only main downside would be the learning curve of HOI4 in general, however Old World Blues is fairly forgiving to new players if they do not play one of the major nations (such as Caesar or the NCR).
I personally recommend new players to play such nations as:
the Texas Brotherhood (you have a technological advantage);
New Vegas (you are not the target of many wars);
Lanius's Cohort (you are much stronger than your surrounding neighbours, who you will try to conquer).
Though it may be recommended to watch a few tutorials of HOI4 before starting (Bitt3rsteel has an amazing beginner's tutorial on his YouTube channel).
Link to the mod: https://steamcommunity.com/sharedfiles/filedetails/?id=2265420196
I HEARD IT BUT THERE AREN'T COUGARS IN GARDENS! THIS- THIS IS MILLIONS TO ONE!