I am adding a car that Yamauchi himself mentioned but was never spoken of again. On a Tweet dated 24 Jun 2010, he made this post on his twitter: フェラーリ365GTB/4デイトナのデザインは、たった10週間で全部をやったそうです。すごいですよね。R T @stratosbaka: @440mashiro @Kaz_Yamauchi フィオラヴァンティー来ていたのですか。。。
It mentions the 365GTB/4 Daytona being modelled in only 10 weeks. I know it was somewhat difficult for cars to be added as DLC in GT5's architecture so maybe it sat on servers like the Shelby Cobra Daytona did (that car was modeled around the same time as the 2010 NASCARs, watch the NASCAR special on Gran Turismo TV and you will spot the exact Shelby Daytona used). I would not be surprised if this car finally appeared in GT6.
The tweet in question was mentioned here:
https://www.gtplanet.net/forum/showthread.php?t=128579&page=26 as well. I am going to be busy digging through what I can on twitter as of now.
Funnily enough, I see a Tweet dated at 25 Jul 2010 about the Alpine A110 being "finished" and Alpine has a massive following in Japan's classic car world. It seems a fair amount of GT6 premium models were in existence already.
Did some more digging. All of the tweets around this time period are all around the time of the 2010 Tokyo Concours D'Elegance (which oddly enough faded from existence afterwards).
http://www.concours.jp/
Notice a hard to find car that was also in attendance. The Ferrari SP1, GTPlanet's least favorite Ferrari, but an important model nonetheless.