A few small points. Firstly, it was already in the Gran Turismo series, and GT Academy drivers drove it (and its successors) in races. It's not a car of which Polyphony Digital is unaware.
More importantly, it was in GT6 before launch, then removed, then put back in again (alongside the Mazda-engined car) after an... interesting bit of legal disagreement. Given that and the reported $1m fee for licensing, I doubt we will ever see it again:
The unique DeltaWing race car is mysteriously removed from the official GT6 car list and game screenshots.
www.gtplanet.net
But atop all that, we have a Cars/Suggestions forum. That's the place to fill up with car suggestions (and its sibling track/feature forums), hence the name: