Other than throwing a few original paint schemes on a car how does one take this game to the next level in personalization? I’m sure there are a few of you out there that have been wondering the same thing. How would you make your favorite car truly your own?
You could change the rims and size of the rims. And also some cars you can change the body kits. But other then that and paint, I’m not sure there is anything else you can do.
Make a license plate vinyl. Always nice to have your gamertag or nickname on the back. I had to make UK license plate and lettering from scratch for Forza 5, after carrying them over in previous Forza’s, but it’s really easy and took about 30 minutes tops. It’s quick and easy and can get you a nice plate! 
It’s a bit of a “wish list”, but I’d like to see Interior customization and mods.
Building/Tuning your own cars and designing your own liveries is about the best way of making a car unique.
Make your paints as if you were designing a car you would actually want to race in. Throw your name up above the door and drive it proudly.
As far as build goes, there seems to be a kind of formula to upgrading cars for leaderboard or lobby competition, but there’s no reason you can’t experiment with different engine and aspiration or even drive train conversions to come up with a unique Frankenstein inspired build. It may not be competitive but it might be fun to drive. And you may get better at tuning by experimenting with the system.
Of course, one has to stay within the limitations of the upgrade system, but there is a large degree of personalization available.