I am huge fan of all GT Original Courses and I will agree how *legacy* courses are much better than those in GT5.
Original Test Track, SSR11 underpass version, Tahity Road, Tahiti Maze, Tahiti Dirt Route, Grundelwald, Midfield, Apricot, Rome Night, original Rome daytime, Smokey Mountain, Swiss Alps, Complex String, El Capitan, Citta di Aria, original Chamonix (asphalt, snow, ice), New York, Seoul, Hong Kong, Cathedral Rocks, Seattle (even Short), Red Rock Valley, Grand Canyon, Paris Night & Opera, Costa di Amalfi... even Motorland.. all of those are PERFECT race tracks and I really hope we will get majority of them either in GT6 or through GT6 DLC.
Original Courses are something that should be transferred through generations of series and I hope PD will deliver on that area.
BTW Despite it was too dark to be driven, one track (course) from GT5 have a special place in my book. It is the original Toscana Night challenge from World Tour from release-version of the game (driven with Lamborghini Murcielago). It is basically a point-to-point track, but it ends in the same straight where it begins I think - and since it is basically a Course Maker course, it was not available anywhere elase in the game.
That particular layout and design was PERFECT IMO. Track had perfect rhytm, dynamics and flow. I would so love to be able to get in in some form in the future as standalone course, preferably with option to choose time-of-day.