A generic version of the Dunsford Aerodrome (Top Gear Test Track) would be something I would approve too, it'll please the Top Gear fans of GT without upsetting the rights currently held by Forza 5.
If they do, they should also at least add the circuit race variants (almost similar to like what Forza 4 had, adding the drag strip would be a plus.):
East short (Follow-through to Hammerhead - Start after Chicago Curve)
West short (Gambon to Follow-through - Start at original starting line)
High speed version (No Hammerhead Chicane or Follow-through)
Also in the Hot laps, the cars should also start at a stop at the starting line (Similar to goodwood's hot lap)
Forza may now own the rights for Top Gear content, but GT still has The Stig's official uniform in BOTH the Black-Suit and White-Suit versions (guess it was kept in GT6 because the suits didn't have "The Stig" in it's name).