Here were my ideas regarding online balancing and car grouping in a future GT game:
-For a given event (or in some cases, a tournament), you will be able to either bring your own car, or in other events, you'll be provided a car tuned by PD, not too unlike what we see now in GTS. However, if you're able to bring your own car, it will need to be stock, especially if the tuning system from prior games returns, where you can add a variety of parts to your car. I'd personally like to be able to add external parts like in GT6, but I think this idea would allow the online ecosystem to be balanced as a result. However, there could be Time Trials, where you're limited to a single model, and you may be forced to drive it stock in some cases, but in other cases, you can outfit it however you'd like before hitting the track. You also could permit modified cars within lobbies.
-For the aforementioned events and tournaments, there will be two parts to the car car regulations: one that mimics an event from the campaign as for what models are permitted, and an N-Series classification. For example, there could be an MR Challenge event that uses lower-level N-series cars like the Toyota MR2, whereas a higher-level N-Series MR Challenge may permit cars like the McLaren 650S or Ferrari 458. This would also allow the campaign to serve as practice for the online, and some events could be repeated in the campaign (like in GT3), except instead of merely having harder AI, they'd use more powerful vehicles that still fit the event's criteria.
-On top of that, there could be additional rulesets seen in the campaign (and online) for the major racing groups. So for example, for Gr.3, you could have event for FIA GT3 cars, GTE cars, and/or GT300 cars, in addition to an all-inclusive regulation for any and all Gr.3 cars. Similarly, for Gr.1, there could be campaign events for Group C cars, modern LMPs, and VGTs, with the last one potentially being a re-tooled "VGT Trophy" event, rather than being a Gr.X event.
-There could even be tools for players to create their own regulations and events, including which cars may be permitted, or what tracks are used, with how long each race would be, and so on. Each created regulation could be given a name, and then shared within the in-game social media system, not unlike a livery. I would hope tunes could be shared in a similar manner, too.
