I'd like to see more championships for the different series. Even though it wasn't officially said, we still got race series in GT4, as the cars (well, some of them) were there.
We got a good showing for LMPs and WRC, the former getting several championships - not least an actual 24h event at Sarthe itself - and the latter getting it's own bloody hall!
But as for Super GT and F1, how many championships did we get for each? One. One championship. I mean, yeah, if you loaded up a Super GT or F1 car in arcade mode, you would get a grid full of SGTs or F1s respectively. But I'm talking about proper GT mode events.
I actually had a weird GT dream last night that made me realize this. In my dream I was playing GT4... which, in my dream, was a much bigger, better version. The only thing I can manage to remember was a screen. It had the black JGTC Subaru Impreza rolling along Special Stage Route 11 (!) as a background, and there were various windows, each with their own events. This was the Super GT hall, apparently.
There were events like 1000 miles, all-night endurance, Super GT World Championship, Super GT street circuit tours... all these different kinds of events, but all for Super GT cars.
And this is what we need! Different halls for all the seperate race series... i.e Super GT, WRC, NASCAR, etc. with their own endurance races, race series, championships.