So.
Thus far we've had... 95 cars across 25 content updates. That's a mean value of 3.8 cars per update. Most updates (15) have just three cars, so we can expect 3-4 cars.
However, every time we've had an update with five cars following an update with three cars, we've had another five- or six-car update. This has happened twice - 1.26-1.29 (3, 5, 5) and 1.46-1.49 (3, 5, 6) - with 1.52-1.54 being a three-car and a five-car update. That means another five-plus-car update would be within established patterns.
The mean type/origin/newness of the cars has been:
Road Car: 2.8
Race Car: 0.8
Tuned Car: 0.2
Asia: 1.7
Europe: 1.6
Americas: 0.5
Brand Central: 2.3
Legends: 0.8
UCD Only: 0.7
All-New-to-GT: 2.6
Returning (including variants): 1.2
As a fun fact on American cars, we've seen a pattern of their presence in updates of 0, 0, 0, 1 four times. 1.54 brought us the fourth repeat of that pattern, with the "American" Escort RS Cosworth, and actually the second instance of them being back-to-back: 00010001 occurred in 1.23-1.34 and 1.42-1.54. We've only seen three occasions where two successive updates have added American cars...
We can therefore expect a four-car update to be three road/one race, two Asia/Europe, two Brand Central and one each in UCD/LCD, and three new-to-series and one returnee.
If "1.55" brings the Hyundai IONIQ 5N and Gran Turismo F3500-A, and we assume a four-car update, that leaves us with two European road cars: one Legends, one UCD-only; one other all-new car, and one returnee.
So... let's say...
BMW M1 1979 (Road, Europe, Legends, All-New)
Gran Turismo F3500-A (Race, Asia, Brand Central, All-New)
Hyundai IONIQ 5N (Road, Asia, Brand Central, All-New)
Peugeot 406 Coupe '98 or Volkswagen Karmann Ghia Coupe '68 (Road, Europe, UCD, Returning)
Because why not.