With Sony lately letting devs do their own thing. the only one that comes to mind that didn't is polyphony. now they are well known for GT and they will probably have that as their main franchise for a very long time. as they are involved beyond the game on real projects with racing and they have multiple partnerships with car companies. 

however I'd love to see what else they would do. a WW2 airplane sim/fighter . or literally anything that has to do with vehicles. 

or a naval combat game.  space combat game (Omega boost)

I feel like 6 and GTS didn't live upto the previous games and GT as a franchise isn't as big as it used to be. it would be smart to let them also work on something else while making GT a once-a-generation game where it constantly gets updates. 

Polyphony still has too much left to do with Gran Tutismo to move to other projects. Within GT they can still add Drag Racing, Formula 1, NASCAR, Rally Cross, Dirt Bikes, Road Bikes, ATV's, Snowmobiles, and much more.

I would rather see Polyphony focus on making GT the ultimate home legitimate digital racing. A Utopia for motorheads of all walks. A place where people can race for real rewards, a place where people can hang out with others that share their interests, a place where creators can show off their mods to the motor culture.

In its ultimate form GT should be the Hub of all things digital and real world. Buy, sell, trade, learn, teach. Want to buy, sell, or trade a car, bike, parts, etc. Gran Turismo should have you covered, digitally and physically. Want tickets to a racing event, car show, or other motor related event, Gran Turismo. Want to learn about motor history, take classes on motor repair, learn to run your own shop or sales lot, Gran Turismo. Want to teach about a motoring subject, customization, restoration, etc. Gran Turismo. That is what I see as Gran Turismo in its ultimate form, like I said Motoring Utopia.

No, but San Diego can and should.
A boxing or skating game pls.

I'd like to see an open world Gran Turismo that takes place during a music festival.

oddly specific 

I'd like them to make a Shuhei Yoshida simulator

Brohei Brohida 

Gimme a Kaz and Shu buddy cop game.

you mean Rhythm game 

LOL, we already have that its called Forza Horizon.