I could babble on about the cars but why do so when images can justify this? There are obvious reasons why the Prototype C mod was the most popular of all on F1C and certainly drove rF1 sales, and why the GTP mod dominated the world in NR 2003. From ISI it'd be incredibly difficult to imagine such licensing to be achieved but one can dream of fantasy names - we have the Howston after all!
After Formula One, I think of Le Mans. Based on how how good the Howstons are, and how detailed they are .. both inside and out, if ISI can't get the licence then moderately inspired generic group C cars would do .. for me at least I know it's not the same as sitting in the gorgeous Mazda and its rotary engine, but if it was similar ( power, weight, torque, downforce, sound .. etc ) then it could almost do the same job as the real things. I really enjoyed the FISI 2012, Howstons and any other car ISI have made, as long as they feel, look and sound similar or have their own character and charm, licenced or not, they'd be a great addition to rF2 and probably a large incentive for some to get interested or / and purchase rF2, I'd hope ( I would ).
Take a look at the Mak Corp Group-c MOD. There should be a new beta in a month. At the moment just the Mazda 787B BUT there is a Porsche 962 and a Sauber C11 on the road.
I can't object with your choice, I still enjoy a lot my RF1 Group C mod. Imagine how skinners could decline all the liveries of the 956 and 962... May be one day...
We need a historic group C car, no doubt. I wouldn't care if it was called 'Fluffy Unicorn and Rainbow Puppy Racing'... even without a proper license, it would still knock everything else out in an instant.
I'd pump out the Rothmans cars, Momo and a few more (for Porsche and other chassis). Sabre, Tomoya, Ferdinand, Rover, Parisien, and Yokohama.