There have been several excellent ideas in past GT games that would seem easy to implement into GT7, such as Shuffle Racing, Car Gifting/Loaning, custom gauges in cars, Event Synthesiser, or Track Voting in lobbies, to name just a few. Indeed, having more features (such as the ability to share Custom Races among players) can only help a sandbox style game. There are so many open goals for PD to score to make the fans happy aside from sharing Custom Races. Heck, on the topic of GT7 Custom Races, PD would make SO MANY people happy by allowing players to disable rubber banding, making it so that the AI use full throttle, and for opponent cars to be able to use more than just the cars' default tyres and Racing Hard tyres. But for some reason, they won't.
For as obtuse and tone–deaf as PD often appears, they are very quick to rectify big issues, such as credit exploits and BoP errors. I want to give PD the benefit of the doubt and say that they're competent professionals who know what players want, but for some reason or another can't implement these oft–requested features into GT7. As is evident by now, PD aren't very open to communicating directly with their fanbase, and so suggestions and discussion (some more obvious than others) can often feel like a waste of time and brain cells. I can only hope that PD realises that this is the sort of culture they're conditioning in their most invested fans by continually being so opaque and silent.
But hey, seeing as I'm spending hours trying to balance my own Sophy grid, I'll chime in with my two cents.
If Custom Races can be shared between players via the Community feature in the same manner as Styles and photos, then I imagine the most straightforward implementation would be much like what PD does with some of the Daily Races held with Specified Cars: players will be given a choice of pre–tuned cars with locked settings and premade liveries, so they can only pick a car and race it as–is. Even in this highly restrictive state, I think there would be a lot of excitement garnered. Many users in the Custom Race thread have been sharing setups and event settings that provide good racing, and I can forsee more skilled players flocking to the feature, revitalising the game for those who find the Single Player Campaign uninspiring.
The complicated bits arise when one wishes to substitute their own tunes and/or liveries into the grid. Semi–permanent car modifications—such as engine balancing and mass reduction—can be temporarily toggled off by Balance of Performance, and so it should stand to reason that the only reason these modifications have any permanence to them is that they are due to Kaz's wishes, and I think we know by now how stubborn Kaz is, for both better and worse. The only truly permanent mods are engine swaps and wide bodies. It wouldn't make much sense to selectively mix and match removable and permanent mods on a car for a Custom Race.
Also, I think it's worth considering what effects sharing Custom Races can have on the in–game economy. Some have already expressed that being able to drive expensive cars without having to own them prior deincentivises players from spending the time to earn the credits to buy said expensive cars, but to play devil's advocate to that, I think being able to test drive these cars in Custom Races can be a good advertisement for the car. One might be on the fence regarding buying a 20 million credit McLaren F1, but they might like driving it enough to then buy one for themselves to tune and/or create liveries for. Of course, cars like the McLaren F1 are very few and far between, and the most expensive cars in the game aren't worth a hundredth of their asking price, but this is still something worth considering nonetheless, given how involved PD seems to be in advertising their partners' products.
And then we get to Custom Race exploits to earn credits quickly. There have already been some
videos documenting how to earn around 1.6 million credits in an hour with very little effort, and should Custom Races be sharable, I would expect these credit grind races to dominate the trending section. I don't know if we players really want to draw Sony/PD's attention to these exploits, because they've been quick to fix these exploits in the past.
Just my two cents.