I like to throw a lot of **** at Horizon Customs, Extreme E and Donut Media Part 1 for this sorta thing--but the truth is, this is exactly the point of Forza Horizon, compared to Gran Turismo. D-class and maybe C-class cars exist precisely because they're meant to be the baseline for upgrade builds; once you get into B and A class, and especially S1, you start seeing cars that are already pretty great at what they're designed to do, so you don't need to upgrade them nearly as much. With a D-class car, you have a crappy car that can become literally anything you want, so something like the Fulvia can easily become, say, a Group B rally car or a GT legend with enough imagination and time investment.
If GT7 added the Fulvia, for example, it could only be tuned for asphalt racing. FH offers these quirky D-class cars as upgrade templates more than anything else.