Yet we can use evidence to guide us.
For example, the average (mean) content update contains 3.6 cars - but that's skewed by a couple of five-car updates (one of which could be classed as a four - 1.29); eight of the 13 updates have been three-car updates, so three is more likely.
Cars are overwhelmingly more likely to be European and Japanese - at 47% and 42% respectively. In fact only five updates have even contained American cars, and we've only ever had two in a row once. As the last update had an American car, this makes it more likely to be a two European/one Japanese update.
Updates are twice as likely to contain road cars as race and professional tuned cars (both of which were SEMA cars) combined. That makes it more likely be two race cars/one road car.
Cars in Brand Central are slighly more likely than cars not in Brand Central (55:45), and notably we've never had a mix of Used/Legends; it's always been either/or. Legends are more likely, coming in seven updates to Used's five, but both times we've not had a Legends car we've subsequently had another update without Legends cars and we didn't have a Legends car last time out (although the two cars in the update were listed as Legends in the patch notes but appeared as Used). This means two BC and one used is most probable, but it's a toss-up.
All-new cars swamp returning cars by 3:1 (with a couple of debatables in there; one of the S14 Ks is technically all-new but it's a struggle to claim that with an entirely straight face. One might also say something similar about the recent Giulia), but entirely new lineups are rare. Interestingly, returning cars have never outnumbered all-new ones; we can thus imagine it will be two all-new and one returnee.
That means we can anticipate a three-car update that consists of two European, two road, two Brand Central, two all-new, one Asian, one race, one Used, and one returning car - but of course that doesn't necessarily means two all-new, 2001+, European road cars and one returning, pre-01, Asian race car; they could be any mix.
The "leaked" list is not a terrible guide for cars which have been worked on for inclusion in GT7 (though you have to keep in mind that - particularly the ones from previous GT games - they are not a confirmation of anything).
Someone pulling three cars from there that match up the above criteria is doing a decent job of predicting. For example if one were to predict:
Lexus LFA (Asia, BC, returning, road car)
Bugatti Type 35 (Europe, Legends, all-new, race car)
Audi R8 V10 Plus '16 (Europe, BC, all-new, road car)
They're using evidence well to make those predictions.
However we should also be aware of a handful of cars likely to come to the game at some point: the Phil Robles Civic, the Pebble Beach Ferrari 512 and Autobianchi Bertone (probably too soon for either), and the Dai Yoshihara Tesla Model 3 PP.
