I voted for lobbies, penalties, and other. Unpopular opinion, but I literally do not understand the outcry for anything else on that list.
Selling cars, why? So we can sell cars for a quarter of what they're worth? If prize cars were repeatedly winnable and sellable for credits, sure, otherwise, it's a worthless feature like the UCD.
Single player content. I hated the campaign of GT7 because the AI is slow, unsafe, and doesn't even play by the same rules as you do. The whole campaign of GT games has been starting last, bashing your way through the field of slow AI before you get bashed for not bashing, and then spending laps 2 to final lap catching the "rabbit". Some of these cars even pit on the last lap of the race with ample fuel! What is the point of this charade? Why do people want more of this?
For me, lobbies and sport mode penalties are the most pressing issues PD need to focus on as soon as possible. Entire leagues and playerbases are staying away from GT7 because of the lobbies - I know, I'm part of one. The importance of sport mode should be self evident, because it's what garners the most attention with content creators and official events.
For a driving game and a self proclaimed simulator, there are still a lot of things about the physics of the game that need looking at, chief among which are the tyre model and FFB, both of which have entire threads opened about those issues. The clutch is still an on-off switch, which makes manual gearbox cars, i.e. some of the best driving machines, slow and jerky, and the manual gearbox upgrade in the tuning shop a missed opportunity.
Aside from this, the single player side of GT7 needs an extensive overhaul to its economy and AI. Make the special parts and engine swaps attainable via hard work and talent, not via sheer, dumb luck. I can honestly say without exaggeration that I've had the most fun with any GT game in the past two weeks designing, tuning, and testing cars, all because existing options opened up to me courtesy of the bonanza. If PD can keep those options open to players while steadily adding more engines and compatible cars to receive those engines, I think they would have a low cost, low effort, easily sustainable plan to keep player retention high.
I'm not holding my breath, though. A lot of what I mentioned has been synonymous with the series for more than a decade now.