Without the Porsche license, my preferences are limited a bit, but I'd love Capris v CSLs v Opel Commadors from 1973, ideally with Escorts v 2002s v Alfa GTAms as a secondary class.
If Porsche license was available, the early DRM years would be suitably fantastic, such as 1977 with Escorts and 935s supplementing the 320s we already have.
I'm not such a fan of the 80s and certainly not a fan of Super Touring (although I loved it at the time, I don't think they've aged well), although I wouldn't complain if we saw the Jaguars and Rovers (maybe just a TWR licence!) to add to the 6 Series and Volvo 240s.
Other than that, I'm a big fan of the modern series approach and would love to see the Blancpain Endurance/Sprint licenses realised in R3E.
Bring back GT Legends! All cars and all tracks...and yes, I know about the Porsche/EA issue. I can still dream, can't I?
I like the early 90's BTCC. Both cars and tracks.
Bring on the V8SCs!
Would be nice to have some series for small bore cars like the MX5 Cup, Clios, Minis (both new and old). These series can be lots of fun with a good bunch of drivers that race clean and have similar pace.
Can Am cars. They would be nuts!
The sky is the limit, really.
As a matter of fact, I agree with just about every post that I've read so far. I am really loving this Physics engine, FFB and track feel. There's lots of tweaking to do to get it to feel just right depending on what the user's tastes are but that's a good thing in some ways. Besides, it's early still and they've made some great progress recently. I'm feeling very positive about this SIM.