I sort of think GT4's campaign could've be trimmed down just a bit. Some events either seemed too long or seemed like they were just padding. But with this in mind, I really want to see the return of events you can enter from a brand's menu, as well as a general campaign design that may not force you to literally get every single car, but does encourage players to drive an incredibly wide variety of vehicles. For example, there could be multiple Gr.3 events - one could include all Gr.3 cars, while another may focus on FIA GT3 cars, and yet another could only permit Super GT300 cars. I believe GT6 did indeed use those latter two regulations in its campaign, as short as it was otherwise.
EDIT: I also hope that the list of typical opponents and the list of eligible cars for each event is one and the same. I'm not a fan of being capable of picking a highly over-powered or under-powered vehicle for an event when none of the AI drivers will use said vehicle. For example, in the Electric Circuits event in GT6, the Fisker Karma and Tesla Roadster are both eligible, but are not used by the AI. Instead of being allowed to use an underpowered model, I'd much rather see something similar to what we see in GT Sport, where you might get an experience/credit bonus for using an otherwise eligible car with lower power, PP, etc. than what is recommended for the event.
EDIT2: I almost forgot to add, that if the Premium Sports Lounge returns, I want more of an emphasis on snobby, "bougie" cars, like Aston Martin or Ferrari. I'd also have both raise the minimum cost for an eligible vehicle, and limit it to certain models, rather than simply permit anything and everything above a certain credit cost. The next iteration of the PSL better have a round at Ascari Race Resort, too!
EDIT3: I had another thought. I'm admittedly a bit divided over the famed "Race of the Red Emblem" being replaced with the "Nissan GT-R Cup," as I don't think the R35 GT-R actually has the famed red emblem that's mentioned in the event's name. But on the other hand, I really would like an event where we could still use Skylines without the GT-R label like the R30 and R31, or the much older Skylines like the BLRA-3. Ah, maybe those could be eligible for some event that features a certain decade of Japanese cars or something instead.
EDIT4: Yet another note. If you look up the "GT-R Maestros" event in GT6, it allows for pretty much any iteration of the GT-R R35, including racing versions. But if we had that event in GT7, I'd personally limit it to the GT-R '07, GT-R Spec V, the GT-R Black Edition, and the GT-R Premium. I also might permit both the 2014 and 2017 iterations of the GT-R Nismo, too, but even then, that would only be with a PP limit. Alternatively, neither of the GT-R Nismo models could be eligible, but one of them could be offered as a prize car. Or maybe even both of them could technically be prize cars, though you'd only get one of them. I generally like the idea of multiple prize cars that follow a certain motif, which would combine the much more random prize cars of GT3 with the themed prize cars of GT4. Some events could still just have a single prize car, but others could have two, three, or four prize cars that all fit the theme of the event.