PD should make a new game like this. 

maybe after gt5 they might, would be good to see something else from them, have they made any others?

The creator of PD and the GT series (Yamauchi) said he'd like to make an RPG in an interview 1 year ago.

But I can't help but think SONY might pressure them into making GT6 next.

All they know are cars... Thats their passion. Although I can see them trying to challenge themsleves to do something else, I also wouldn't be upset If they just stuck with vehicles. Let everyone else do everything else, no one does cars like they do. I could see them making a mech game or flight game or something of the sort

Well...I wouldn't mind if an entirely different team handles the RPG, but with Yamauchi having creative input into the game. It may be strenuous overseeing the development of two games, but it would be great if he could pull it off.

A mecha Game like Omega Boost or something else could help Yamamuchi san might find making GT easier because if you focus not just on one thing like GT your bread and butter project like GT could benefit from it. 

I wonder what kind of rpg would PD make? Maybe something set in the future I guess.

An RPG based on Omega Boost! ô/