Let me put on my speculation hat. Assuming the Monaco build already had all the upcoming content there, it's safe to assume any other cars will not be from brands already 'leaked'.
The P1 GTR will be the only Mclaren. In the dealerships, only 6 slots appear on-screen, at a time. Also, cars are ordered, from left to right, least expensive to most expensive. Bar a classic Mclaren, the P1 GTR will be the most expensive one added to the game, so far. Now let's look at the dealer, with 4 slots visible on the picture, leaving space for two more slots off camera. The F1 is the rightmost one and there's one more car to it's right, the VGT. The P1 GTR will cost more than either of them, so it has to be the car on the 6th and final slot. As much as I'd like the road P1, it isn't coming because there's no way it would be priced higher than the GTR, and there's no space for it in the dealer.
As for Chevrolet, BMW and Ferrari, imagine someone browsing the dealers, seeing a new addition and saying to themselves: "Oh look! A new car is here! I am not at all curious if there are more new ones around here." No one will think that way. They will scour that dealership for new cars and the only reason for them not to find any others, is simply because there are none. I don't believe for one moment that they would leak one new car, and not the others, that they would have easily found in Brand Central. If someone is brave enough to spill some beans, and can get in trouble for it, then they certainly wouldn't have reservation about spilling the rest, had they found more.
So, in conclusion, let's focus speculation on brands not already revealed to have a new car.