We've known this for a long time... GT is a jack of all trades, master of none. But slowly and steadily, the game gets better and better over time. and eventually, it can become the master of many trades with the more that it adds to its repertoire.
I can appreciate the drift events, car customization, concept vehicles, as well as the GT class offerings GT has on offer. I can also appreciate a livery editor, track editor, and an open world/city area or gymkhana area if they were ever to be added.
I don't know what the lot of you feel Gran Turismo is actually about, but personally, I feel that anything that is vehicle/high speed driving/racing related, all fits into the Gran Turismo category. Why put a limit on GT? Why put a cap on how far GT can go? Why not allow GT to evolve to a higher state that could allow more people to enjoy it in a grander way? I don't understand. What are y'all so afraid of? What's keeping you from allowing others to have something that they would enjoy in a game that they love? You can and still will also, I assure you. If you don't like bikes, don't use them. However, I can guarantee you, that even if you do not like bikes. You would still end up trying it out, tinkering with them, and possibly even liking it over numerous of the car/vehicle offerings that GT has on offer.
Take a chance. You never know what could come of it. And as I can recall, it has already been done. Gran Turismo HD is actually a concept for Gran Turismo on the ps3 that had both motorcycles and cars in it. They did not call it Tourist Trophy, they did not call it anything else, other than Gran Turismo. It's still and always will be GranTurismo, and just because it has bikes would not make it any different.