I believe I may have come across some more cars that might end up in GTHD (or at the very least, GT5).
This news item is on the Gran Turismo Japan website:
http://www.gran-turismo.com/jp/news/detail.do?article_id=369
The link points to the Motor Sports Japan 2006 Festival in Odaiba:
http://www.motorsport-japan.com/index.html
This even, as you'll notice, is sponsered by Gran Turismo. You'll also notice a lot of cars from previous Gran Turismos are actually at this event:
Honda NSX JGTC
Honda NSX GT2
Nissan Fairlady Z JGTC
Nissan Skyline GT-R JGTC
Nissan R92CP
Nissan R390 GT1
Nissan Skyline GT-R (KPGC-10)
Toyota 7
Mitsubishi Lancer 1600GSR Rally Car
Mitsubishi Pajero Rally Raid
Subaru Impreza Rally Car
Mazda 787B
McLaren MP4/4-Honda
Lotus 100T-Honda
Williams FW-11 Honda
Some notable cars on the list that I think we could possible see in GTHD if PD did in fact test some of these cars:
Toyota Celica Rally Car (ST165, 1990)
Toyota TS010
Toyota R85V
Suzuki Swift 1600 Rally Car
Nissan R381
Nissan R382
Mazda 767B
Honda RA301 (F1)
Honda F107 (F3)
Dallara FN06 (F.Nippon)
Lola T97-51 (F.Nippon)
Courage LC70-Mugen (JLMC)
Zytek 05S (JLMC)
Ferrari 550-GTS Maranello (JLMC)
C-West Advan Fairlady Z (JGTC)
etc...