God I hope so. I loved the track editor in GT5, as primitive and crappy as it was, those short test courses were mine. I'd create courses specifically to test tuning instead of using High Speed Ring or Trial Mountain to test.
If it was to return, I'd hope to see it more fleshed out so you can make a narrow, winding Touge run or a mock-FIA race track to everything in between. More environment settings, finer controls, more control in general for that matter would be a blessing.
Apart from losing everything in GT6 when the servers shut down, I wasn't able to create tracks anyway 'cause I've never owned and likely never will own a mobile 'phone (can't use them, long story). Went mad with it in GT5 'though and loved it even 'though it was pretty-basic.
Been drivin' GT6 now for 9 years and for sure those extra, missing tracks would go a long way towards keeping me entermatained until GT7 arrives. If PD requires an external app / 'phone for this in GT7 I'll be beyond-bummed. Just find a way to implement it in-game using the controller and I reckon most will be very-happy.