FR cars don't "dominate" over FF cars, certianly not ones with as much front downforce as the RM cars have. I'm not sure what class it was, but it was the lemans series on speed the other night. They had BMW's racing with mini's and golfs. If the cars are setup fair, and with your weight penalties, how could they "dominate"? Another option is not allowing the top 3(last season) to change cars, or only allow the last few to run FR's.
I love both of those cars Jav mentioned and would join in one of them in a heartbeat, and with two more cars you'll have less DC5's.
The independant classes were unpopular because those new cars are far less exciting then the original 3. Also, many are running the same car as last season so those racers have a huge advantage in car setup. By adding some hot FR cars you make the second season exciting...how will the new cars fair against the old ones? who will take on a new car, and who will stick with what they know? etc
Your idea of adding cars was great, however IMO those cars were just not the "mix up" the series needed. However, i could be wrong. Maybe everyone is ok with doing the same season over.