Not sure if it's small - it may only be possible in the next GT - but I'd genuinely love to see more classes be part of the Manufacturers Cup, especially as we see more and more brands join LMH/LMDh in real life. Perhaps it could even include road cars.
Said road cars could be from all across various eras, and would be pre-tuned - perhaps even with engine swaps in some cases - but would all be roughly the same PP (perhaps 600) and races with them would use the various Sports tire compounds. Of course, they'd likely have notably different characteristics, just as we see in the actual categories like Gr.3. For example, I had this swapped Honda S800 that only has 222 HP, but it weighs so little, it feels incredibly fast when used for various WTC 600 events. Heck, upon completion of the season, you could even be gifted the pre-tuned road car you were assigned for your manufacturer.
EDIT: I suppose one question would be whether the road cars should be relatively random and then tuned to about 600 PP, or if the models should actually come with around 600 PP as-stock and then perhaps lightly tuned if necessary? Or could there be multiple road car options for automakers, like we see with the more formal categories? We all know of the many Gr.3 options for BMW, or the various Gr.1 racers you can get from Audi and Toyota. In this regard, some automakers may only have one road car that could really work, like with how the GT by Citroen Road Car comes with about 600 PP as-stock, but I don't see the brand having another model that could reach 600 PP. (Unless a swapped DS 21 could do so.) Meanwhile, Honda could have both that aforementioned swapped S800 and the 2nd-gen NSX.