I don't know if it's already been hypothesized ITT, but I had some notes!
1. I think the best idea for a mobile spin-off of a given IP is to have it occupy its own niche rather than just be a portable version of the game. See: Pokemon GO vs. the mainline series.
2. Given this factor, I think some sort of connectivity with the "main" installment would be a great incentive to play the mobile version, too. Again, Pokemon GO is a good example, where you can transfer (most) specimens you've caught in GO to the Pokemon HOME app, and then move them from there to a compatible mainline game, such as Pokemon Sword and Shield.
3. With the above two factors combined, what if "Gran Turismo Mobile/Superleggera/etc." is actually a B-Spec app? I'm normally disdainful of B-Spec, but I think the mobile platform would be a very nice fit for the B-Spec mode, rather than inserting B-Spec into GT7 itself. Especially since B-Spec seems like a relatively passive mode, so you could check up on how your driver is doing every now and then without needing to constantly pay attention like you would for the "main" game.
Then there's the factor that PD did have job listings for mobile developers - and this was after the GT Sport companion app was released, too. Heck, even if the mobile version of GT is basically a B-Spec app, I might have to give it a swing.
But one thing I can say for sure is that I think it'd be stupid to have the mobile version of GT just be a scaled-down GT game. Occupying its own niche would be much more preferable, at least as far as gameplay mechanics go. I think whoever makes the mobile version of Gran Turismo - whether it's Polyphony Digital or some contractor - they should take broad notes from how Pokemon GO works with the mainline series, both in terms of their niches and how they literally work with each other with their connectivity.
This mobile version of GT could even have multiple features. Not just B-Spec, but maybe a livery editor you can use in the app, or an enhanced way of accessing the social media component. I always thought the GT Sport companion app was merely the beginning of something greater, something more robust...