I don't think that there should be any bikes in GT6. It's not that I don't want to race cars and bikes, but motorcycles are so different from cars that PD would have to completely redo the physics.
That is absolutely inaccurate perception.
Cars and bikes *shares* the absolutely same physics, what is different is actual physical behaviour of the vehicle/chassis/tyres, but once you have good physics-model, it just brings down to applying driver's weight-shift and dynamic-aerodynamics (which original TT already had) into physics.
Many people do not know Polyphony Digital's motorbike-lover Takamasa Shichisawa, who is normally working on the landscape design for GT games, and one of the guys from studio made first in-house demo for TT in their spare time with some basic help from Akihiko Tan and 3 more PD guys.
They literally made complete game before showing it to Kazunori who was so fascinated by the alteration that he made TT project being greenlighted at SCEI. Because of his work Takamasa Shichisawa was named as Director of the Tourist Trophy game and on the cover of the game it is him who is riding the CBR600 on the photo of the TT cover.
Akihiko Tan - director of the GT physics since the first game - was in charge of physics part, while Norio Takama - guy who was in charge of human animation for GT4 and some support programming - made all algorithms for driver body-behaviour and such.
In short, Tourist Trophy was literally *baby* of 6 people in PD who was working on it in their spare time with available assets and resources from GT4.
Tourist Trophy 2 would not be waste of resources or anything similar by any mean if ever greenlighted. Guys would produce it instead of sleeping, same they did with the first TT - which is still the best motorbike driving simulation ever created.
For actual implementation, I see it as either DLC or *normal* disc-release that would either work as standalone game or allow to be *imported* into GT game. Games would *share* the same title screen, but GT Life would be one section and Tourist Trophy Life another. For the Arcade Mode you would be able to select mixed races, as well as for Private Lobby races in online mode of both sections. Also, it would be great if owners of both games (installed on the same console) would get some mixed Seasonals with nice challenges, etc.
Of course, nobody would be *forced* to anything, if you want to play only Gran Turismo and never to see bike on the track with cars, you could no problem - but if you want integrated games, you would get an option to do so with complete options.
I would gladly pay up to 60$ for a proper TT expansion for GT5/GT6 and wouldn't even think twice.