This is the most recent thread I could find for this particular suggestion. I think this is even more important than it was back when this was originally suggested as GT7 now has the ability to add other racing components to normal road cars, but the problem is that those road cars always retain their original interior. In some cases, this is fine...but in others, like classic cars, it becomes a little silly when you have super skinny, large diameter steering wheels or hopelessly insufficient gauges or bench seats or what have you. GT should have some amount of interior customization. Roll cages have been added, which is great! Here is my list of things that would make the biggest difference for (I believe) the least amount of work, in order:
1. Steering wheels. Only 3 or 4 styles would be necessary, IMO (a classic model like the Momo Prototipo, a "retro" model like the Momo Montecarlo, and a contemporary racing wheel or 2)
2. Racing seats. Again, 3 or 4 styles representing a few eras would be totally fine.
3. Dash/Gauges: Gauges & Info displays would be more difficult to implement, but maybe a generic "race pack" style digital dash could be added to most cars, just in front of the cluster, if so desired, with 1 or 2 different styles.
4. Removing or "gutting" interiors would require a considerable amount of extra modeling for PD so it may not be the best use of time/resources. Maybe for certain cars it could be done.
One of my favorite parts of the old NFS series was the ability to transform a road car into a race car, including the interior. I feel like this has become more conspicuously absent from the GT series as the exterior customization has improved.