There have been several attempts at an intermediate class, and almost all have failed. In the brushed days a local track had a 19 turn class. I personally witnessed a heat where every car didn't finish due to motors failing (overheating). The gearing and timing were pushed beyond the limit, and losing a motor for most racers was a weekly occurrence.
On the national scene it should be 17.5 and open, nothing else. Local tracks should come up with their own slower class (USGT, VTA, heck I know of a track that runs Tamiya TT01's!)
Another comment, I think the recent downturn in racers has more to do with factors outside of RC (economy, etc) than the classes we're running. Racing RC cars will always be fun, but there are many other distractions out there.