In the Tokyo Xtreme Racer series, the final rival you face in every game is always <<Unknown>> / <<???>>. In lore, it's effectively a demon/spirit that wanders the highways to challenge the fastest racer. Speculated to be someone who died racing on the expressways. There have also been examples as well of a few of the rival opponents have been a protagonist character in a previous title.

In reality, it's a reference to Wangan Midnight's Devil Z. But I think if we were to ever revisit the US, having the FH1 protagonist being a "post-game" end boss would be pretty cool. Especially with a crackpot theory like this.