You are really making this sound like crazy.
Being "backed up" by Sony means nothing for actual success of GT series. Yes, without Sony the series would probably have less success than it has in various fields, but it is really no matter of having backup. There are tons of other first-party titles that just couldn't maintain commercial success despite having their backs covered by Sony - especially in marketing.
For "racing" games you just take a short trip to history: Formula One licensed games (even exclusive multiyear licence couldn't help for gaining the sales-momentum through years), WRC games (same as for F1), Motorstorm games, Wipeout series (RIP).. all first-party titles, all heavily supported by marketing, all having insane backup by most powerful SCE division in SCEI - SCEE. And nothing helped them. Because commercial success just didn't happen.
Same is going on "on the other side". Microsoft just decided to kill fabulous Project Gotham Racing despite it was first-party title from their top-tier developer (Bizzare, never to be forgotten). One can argue they did it to *protect* Forza series, but sheer truth is how PGR just couldn't achieve AAA sales needed to continue with funding of the series. It is simple. Unfortunately, extremely poor results of Forza Horizon will also not help Playground games guys (of which many of them are from studios that were making games I've mentioned above) while one can argue about the future of the Forza series overall, because sales results are not raising with time and costs for maintaining a +400 army that produces FM series are probably not low. But time will tell.
To conclude, without commercial success every game will cease its existence. And GT will not be a exception. If it can't make profit, it will not exist. But all until they can be among the top-selling titles in the history of genre (as I am aware, Gran Turismo games have all top spots in genre historically, when racing simulations are concerned) they will produce new games.
As for the question of the thread, I think how GT has tackled many fields that are actually making it a *game-changer* from many perspectives from the first game. I always highlight the overall *ownership/maintenance* aspect as my favourite and
I would love to see further development on that particular field.
But as for one option that is still missing from the series, I would go with
"Build Your Car". We had that option in few simulator games already (Car Workshop in the original SegaGT or in more arcade-like Apex Racing Evoluzione for instance). Gran Turismo already has a magnificent foundation for such option, where call Premium cars are virtually built as the real cars - from actual parts, combined into 3D modelled vehicle. Ideally, those "Built Cars" would be racing models-only, spanning from some small open-wheelers to usual GT-series "Dream Car" category of GT1/LMP vehicles.
In the ideal situation, we would have a choice of choosing a actual chassis/suspension modelling by type (for example 4 dimensions for *touring/GT1* class, 4 open-wheel dimensions and 4 LMP chassis or however), choice of wheels and covers/aerodynamic packages and choice of engines/drive/mechanical parts up to wanted power/aspiration type/whatever. Such cars could be shared among friends and used in-game as any other vehicles. Of course, it would be great if even separate Championships/Cups would be introduced where player would be encouraged to build and use such vehicles within GT Life mode.
I always wanted to GT have a small class of 200-300HP open-wheelers for instance, with small footprint and V6 engines. In the ideal situation, I would choose "open-wheel" MR chassis, imaginary *PD200R* covers and aerodynamic package, semi-adjustable suspension, I would take Toyota V6 engine, racing mufflers and install Racing Hard tyres. I would have an option to decide about the engine placement position for balancing purposes, I could adjust the ballast on chassis, I could choose type of wheels and dials for interior. Voila, my car is done. I share it with my friends and we all have our own championship with that car.
I think such option would provide a great additional value to GT series and would be a true *gamechanger*.