I think the key will ultimately be content and variety available at launch (cars and tracks). Cars looks likely to be GTS+ (maybe between 400 and 600) so I think track numbers will be the key component.
One thing I would do is amalgamate the different types of events into the career mode as a single whole. So, for example, in GTS you have Drift mode, Time trials, GT league races. Have a career mode that includes all of this but where you have a choice of pathways to the next level, so if you dont like drifting for example, you can still progress without getting gold on those drift events. Prize cars given out when you get gold in all events of a type and when you complete the level 100%.
Add the coffee breaks back in between levels because I think they were quite popular in GT6, but try to think of more things that people will come back to even after they have got Gold. I think the "how far you can get on 1L of fuel" one was a good example as people went back to squeeze that little extra bit of distance out of it, even after completing it. The cones events were pretty dead, once you got 100 cones down that was it.
Single manufacturer events included within the career mode as well.
On top of the career mode, keep separate modes for Custom Race, Time Trials, Drift Trials. So the fans of specialisms get to enjoy both the career and their own separate events without it being at the "cost" of other players.
One thing no one has mentioned yet, how does post launch dlc get included in the career. Although it's very simplistic, the modular setup of GT League has made it very easy to add in new content cars and tracks through new races and events.
I actually like the idea of an "evolving" career mode as, if it is done well, it can help reduce stale ness from repeated playing. The more complex the career mode the harder it is to augment it though.
So, the choice would be to either add new DLC as individual "packs" with their own events or integrating it within the original launch career mode.
The problem is, if you integrate it, you mess up the dynamics of any prize cars etc given in the career. Look how many times we've had to get the Circuit Experience back up to 100% since launch

.
So I think it has to be done separately, but with a lot more thought and effort than has been applied to GTS in 2019.
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