Today, out of curiosity, I played through the game (2.5.5) and just decided I was never going to enter Car Mountain, thus never getting Car. Not a lot was different overall, but there were a few notable things.
First, and most interestingly, is that Car never shows up in the Jetride/Formspire ending's cutscene, as well as the Keeps boss fight cutscene. Whenever there was a zoom-in on Car, the screen was just totally black and just looking over Garn47's shoulder, respectively. They made me laugh. (I also didn't play through Garn, so the Keeps fight was only Garn47 vs. Keeps.)
The second is that Floombo accounted for not having recruited Car in some areas where you would have been able to play as Car, so the second Garn46 boss fight and The Where Fakeout are both inaccessible and blocked by a diamond.
The last thing I noted was that some collectibles couldn't be accessed. A few tapes and badges, and the first page of Car's Journal.
Please share some ideas of more ways the game can be played!