I run boosted 17.5 TC and love it...its what I run at my home track and when I travel. Almost all of the tracks in my area (Southeast) run the same class structure as IIC and Birds and it works well. Having said that:
If we are doing away with boost completely, I'd like to see 17.5 stock, 10.5 superstock, and open mod. I don't see a need for a 21.5 class at the regional/national level. Individual tracks are more than capable of running whatever rules they see fit for a sportsman/novice class. At our track, VTA is kind of the entry point for on-road, but some tracks need a "run what you brung" class, or Short Course on carpet class, or whatever it takes to bring in the new racers. Non-boosted 10.5 is probably a better stepping stone to mod than boosted 17.5 anyway just because of throttle feel. Like EA said, just because the lap times are close between boosted 17.5 and mod doesn't mean the cars drive anything alike. If the point of a super/prostock class is to prepare racers to run mod, then the cars should drive more like mod cars drive.
If timing/boost is going to stay, I think 17.5 spec, 17.5 open, and mod is about as good as it is going to get, at least for touring car. That structure obviously works as it is in place at big events like IIC and Snowbirds, and seems to be popular all over the country. I enjoy running boost because I enjoy tinkering with the speedo settings and trying different things, but I'm kind of an egghead and not everyone likes it. So be it. Lets race
-rocky b