Here's my take on the off-chance of the Touring Cars from GT5/6 returning:
Chevrolet Camaro Z/28 RM - I honestly believe it would be Gr.X to fit with the other Vintage race cars.
Chevrolet Corvette Z06 RM - Gr.3 no doubt. Would rather have this particular Corvette over the ZR1 RM.
Chevrolet Corvette ZR1 RM - Semiclone of the Z06, therefore I don't see it returning.
Dodge Challenger R/T RM - Like the Camaro, Gr.X.
Dodge Challenger SRT8 TC - I think it'd be Gr.3. It's essentially an LM-meets-NASCAR race car.
Honda Civic Type-R EK9 RM - I honestly don't think this car would come back, personally.
Honda CRZ TC - Definitely Gr.4. It's right at the performance level associated with Gr.4 cars.
Lexus ISF Racing Concept - Now THIS would be a great addition to the Gr.3 roster. Please PD!
Lexus ISF RM - Definitely Gr.4, and a worthy alternative to the RCF Gr.4.
Lotus Cars - Unless PD can strike a deal to get Lotus back, there's no point in including these cars, sadly.
Mazda MX5 TC - Definitely Gr.4, and a welcome addition too!
Mazda RX7 TC - Without a doubt Gr.3! Lovely car to drive on GT6.
Mitsubishi Lancer Evo 9 RM - Gr.3 for sure. It's essentially just an HKS CT230R anyway.
Mitsubishi Lancer Evo X TC - Pointless, as we have the Evo X Gr.B.
Nissan GTR TC - Highly doubt this car would return. If it does, Gr.3.
Nissan Silvia S15 RM - Definitely Gr.3. It's got the looks, just needs a buff in performance.
Subaru Impreza WRX STi RM - Kinda pointless, but still, maybe Gr.B.
Subaru Imprexa WRX STi TC - Gr.4 for sure.
Suzuki Cappuccino EA21R RM - Hmm... it would need a serious power increase to be Gr.4, but it might work.
Toyota Prius TC - All joking about Priuses aside, I feel like this could work for Gr.4. Ditch the CVT for a 6-speed stick or sequential though!
VW Golf MKIV RM - Would be a fun addition to Gr.4!
I probably missed a few. My bad if I did.