I Would love them to, but the issue would be how they affect the FIA race classes, the GR's. There is evidence that PP may be making a return to GT7, the values used in previous titles to determine the performance class of cars. To prevent any kind of unbalancing in GR. classes, RM (Race modified cars) may have to be PP dependent rather than class dependent, since the performance of fully RM cars differed greatly. Or then again, what would be more risky is RM cars having their own class, being further divided into sub divisions depending on performance.
This means you won’t be able enter a RM'd car into let’s say a WTC/GR.4/GR.3 only race, since those races are intended for the FIA accredited cars.
So, the only way to race them against FIA cars would be in uncategorized open free for all races where B.O.P. is not a factor, the only restrictions there in being perhaps, power and weight restrictions with a pp cap. That way you could have more accuracy and freedom of just where you intend your RM'd LC500 would fit for example. I'd imagine there would be a plethora of these kind of races, especially throughout the career campaign, GT has always featured run what you bring race formulas with the PP limits correlating with race event tiers (beginner to expert races)
In past GT games where RM was possible, the amount of downforce an RM car could potentially generate was quite modest but still very effective. This makes sense, as it's not like you’re creating them from the ground up, but rather using the shell of the original road legal vehicle. So, making silhouettes or prototypes most likely won’t be possible.
It would be awesome, but we are talking about the hopeful return of a feature that PD can’t seem to decide how it will work and whether to keep it in the series. Creating Silhouette/prototype race cars like that in the image above through an RM feature would be more like the direct result of this feature expanding, had it stuck around.
Not to say that should the I8 make an appearance in GT7 and race modifying it was possible, couldn’t end up looking like something similar, but yeah, we got to be reasonable about this feature returning and what it could produce. But at the same time, history tells us that the RM feature in GT games was always great for creativity such as that.
I mentioned this before in a similar thread, but with this modest performance expectation of RM cars, I'd imagine that it might just be plausible that the maximum downforce an RM car could generate would sit between a GR.4 and GR.3 car. In the end, the FIA race cars would be what you should aspire to, and would be the focus in game. The purpose for RM cars would then be more for fun experimental purposes and other ways to interact with modifying and personalizing cars next to livery editor.
All theoretic of course.
Unlike the real world though, PD should make RM cars a cheaper viable option monetary wise, compared to buying manufactured prepared race cars. Meaning RM cars could be done quite early into your career in GT7 without having big pockets, giving us the incentive. Fully race modifying a MK4 MX-5 may cost you 50k, while race modifying a Gen 1 NSX might cost you 90k etc, the more expensive the base the more expensive the RM service.
Lastly, should racing RM cars in bopped events with FIA cars be possible such as multiclass racing (should that be implemented) RM cars may be heavily restricted in these cases, with high weights and less HP. If RM cars do end up having their own class with sub divisions, it would be a little bit more believable for joint races with FIA and RM cars to happen. As long as they are placed correctly in corresponding classes.
Should grids sizes ever augment as such in GT7, perhaps a way to envision how such a race event with a plethora of classes racing together could function, would be to inspect the super Taikyu and 24-hour series. In both cases the GT3 cars are the fastest machines on the track.