Isle of Man TT circuit would be great, as would any mountain-pass style point-to-point.
It's not really a point-to-point (again) but the Targa Florio would be good!
Ginettoet al. have almost finished the 44 mile (72ish km) Madonie Corto (yes, short version!) Targa Florio circuit for GPL
If it'll work on a hacked-up 10 year-old game, I don't see why it can't be made available as DLC for GT5...
point to point races would not only be a fantastic idea for rally, but what about touring events!... but it takes lots of space to make a point to point races... 20 or more miles of non repititive space