Well, because every tastes are here, will be very difficult to please everybody. I have seen some ideas here and all of them are so different.
On my side, I think I would something powerful but sliding a bit. Something from 1990 perhaps, supertouring... Hard to choose In fact...
And for the track, an F1 track would be something logical with all the the F1 ingame. But it could be something long to build. As Renato said, It could be reasonable to choose something not too complex.
On my side I like Jarama layout...
But lets wait the vote opening
I mean for some series people will do what you did. Find a nice rfactor mod, get permission, update it to awesomeness and release it. I just hope Reiza makes most of the major series. TUSCC, Indy, NASCAR, etc. I mean they basically have all non NA series.
i have the perfect suggestion for you, Australia Touring Cars Master, they are powerful (600 HP) and sliding you will have and to add to that no other games offer that kind of cars so it would be a big plus.