You know what I miss? The US Muscle cars! Only had 1-2 races with those! Also the Caterham CSR200, those cars are easy to handle and perfect for a fun race, just like the Minis, but still something different. And the Formula BMW-s!
And for the format, I really liked the events with 3 short races on different tracks! The only downside is that you have to learn several tracks if you're not familiar to them!
As for the enduro vs sprint thing: I think we need them both! (and not only these 3 hour enduros, but 1-1,5 hour ones as well!)
They're completely different! Sprint races are all about pushing & overtaking, while enduros are more about perfect driving and not making any mistakes! In my opinion finishing an endurance race in a good position is also more rewarding!
Edit: Some event ideas:
1. Caterham CSR200 @ 3 short tracks, starting @ 19:00 GMT, 10 min practice, 10 min superpole quali, 20 min race for each track. Weather: Dry; Tracks: STCC tracks f.e.: Mantorp Park, Valer, Karlskoga
2. US Muscle endurance race @ Road America, starting @ 19:00 GMT, 20 min practice, 20 min quali, 1h20min race, Weather: Changeable
3. Formula BMW sprint races @ Okayama, start: 19:00 GMT, 15 min practice, 15 min quali, race: 2x25 min. Weather: Changeable