RMs (and the TCs since it was found out that they were actually RMs by Sorg) should be done away with in this day and age. Instead we should be able to create said RMs using what is available in the aftermarket parts section of GT6.
...RMs should not be their own cars seeing that they are entirely Polyphony creations based on in-game car models that were already in Gran Turismo 5 to begin with. RMs strike me as another lazy way of jacking up the car count.
Firstly, RM's themselves have never been included in the car count, otherwise GT2 would have had 900 cars or more.
Even though I might be with you in essence, I'm still not, which I'll get to. You mentioned drive trains in the snipped part, which you might have meant wide body kits as are found in many racing leagues, such as DTM and Super GT. This with a Livery Editor really would be a completely user defined Racing Mod, which I agree would be the best way to go. You could choose to have a wide body Mustang, Supra, BMW M5 etc, or the regular axle width, and both of these body types would serve to produce race cars suitable to one of the real world racing leagues.
However, I'd like for GT6 (or 7) Race Mod to include alternate liveries, like we got a taste of in GT2. I think a small number of cars had up to three livery choices. This way, the car count wouldn't be bumped a few cars because of so many liveried race cars. Hide them in Race Mod, so they can be "discovered."
But I also want to also see some Polyphony creations mixed in, so that we can have a full field of cars from various racing leagues - well, as many leagues as possible in GT6. I know that racing teams have to be licensed as well, so I'm fine with fantasy cars. The only thing is, they need a car artist who has a sense of livery creation, like some of the amazing artists which use other games to create wonderful designs, in particular the Forza artists who can make Photoshop quality liveries.