I think a sensible mutliclass server can work, make it STD+TBO+LRF and another server NGT+GTR for example. If you go for too many different servers racers will just spread out (which was a problem for CTRA too) and you will get 10 servers with 2 people on each of them.
Race lengths are quite important in my book, the shorter the race the bigger is the chance you get some of these nasty T1 accidents because people just don't have the time to go through the field reasonably, make it around 15-30 minutes in length and you should be getting better racing.
Track change, the track shouldn't change to often, yes kids these days can't bother driving something for more then 3 races before they get bored but to learn a track properly and to develop a somewhat working set for it you need at least 8-10 races. Hard to say what's the sweet spot there, you just have to try it and see which setting works best, maybe even only do a track change every 12/24h as mentioned already in the thread.