I'll let all of you have another 2 hidden tracks from GT6 that you can add to the list
(Obviously picture heavy for proof).
Both are called GT Arena (Open Layout)
1st one (Literally an open arena):
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2nd one (Stunt arena):
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I might have one or 2 more pictures lying around and I definitely have at least one replay saved from the stunt arena.
The Open arena worked perfectly fine in time trial mode, but with single race it would spawn all cars in a line in a black world like in the first Cappuccino picture, then it would load the place properly, then the AI would all fight to try to get to the center. Also you couldn't take control in single race. When viewing a replay a horrible screeching noise would play on a loop and all the AIs tyres would produce smoke no matter what they were doing, but after quitting the replay and loading a new track, some of the textures would just appear as black such as the first AE86 picture in the Stunt Arena or like:
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The Stunt Arena worked mostly perfectly; correct AI lines, replay cameras that worked, hundreds of hittable boxes that didn't slow the game down at all and obviously jumps and a half-pipe that worked. The only problems with this track were if the AI were in slow enough cars they wouldn't make it over the ramp-to-ramp (which was set up for 60mph for a perfect landing) and get stuck; and if you rolled over or got reset near the half-pipe it would spawn you in the half-pipe, but you could just drive out.
I will try to record or try to find someone to record the replays I have for more proof if wanted.
If there is anything else you would like to know just ask (except if it involves getting to these tracks), I don't have access to these tracks anymore but they are still in the game.
Enjoy!