I was originally working on a very large list of possible cars, but I've scrapped it.
I think it's possible the next content update (leaning towards late March personally) may have content that coincides with GT6's servers being shut down, i.e. some last "best ofs" from GT6 to make the transition for "displaced" players easier. I don't think all of these will come in a single pack, but I think most if not all are highly likely. Here's my list:
Acura/Honda NSX - any one of many older generation models
Chevrolet Camaro - any one of many 60's/70's generation models
Chevrolet Corvette - C2, C5, or C6 (one)
Ferrari Dino
Ford Mk. IV '67
Honda Civic - any one of many older generation models
Honda S2000
Lamborghini Murcielago
Lancia Stratos
Lexus LFA
Mazda MX-5 - any one of many past generation models (though I lean most heavily to a first gen)
Mitsubishi Lancer Evolution - any one of many past generation models
Nissan 370Z (or alternatively, another past-gen Z model)
Nissan R33
RUF - any one of the available models (though I lean most heavily towards the Yellowbird)
Subaru WRX STI - any one of many past generation models
Volkswagen Beetle '66
I personally see the above list as the most likely (if everything were to be a returning model), but there's also a few more I'm on the fence about:
Alfa Romeo 8C - a fairly popular car, but doesn't (IMO) cover anything in GTS that the 4C isn't already covering, aside from a higher N class. Doesn't seem massively productive to add it in.
BMW - I don't know what of their premium crop in GT6 is all that "appealing" for inclusion given the M4 and largely-modern Gr. 3 cars. At this point I would consider a standard>premium upgrade from the selection to be new to the series, but maybe the M3, Z8, or Z4?
Dodge Viper SRT10 - We already have a modern and old gen viper available to us, the SRT10 was mightily popular, but I don't see it being high priority for PD.
Ferrari F430 - Remained popular despite having the 458 at disposal, but I feel like the Dino is more likely and that this pack is more likely to have significant brand/drivetrain/class diversity.
Fiat/Abarth 500 - fairly popular car in that game, but doesn't seem to have a lot of modern relevance/attention right now.
Lamborghini Gallardo - very popular, but now overshadowed by the huracan. Also the Miura, but in general I feel like it's low-priority.
Lancia Delta - honestly I just see the Stratos as more likely.
Lotus - a surprise return with many familiar models. Don't think it'll happen in GTS though due to some licensing problems.
Mazda 787B - I don't see this having a place in GTS yet with the classes currently available - not that that's stopped PD before, just my opinion for this particular car.
Nissan Silvia/S14/240SX - personally just see an R33 or Z as more likely.
Pagani Huayra - one of the most popular cars in GT6, but for some reason I don't see this as likely as some others. Maybe it's gotten a little dated?
Tesla Roadster or Model S - cuz these may not be the most particularly relevant models at the moment, but Musk is all the rage these days.
Toyota Prius - weird one, but lately Toyota's been all the rage in content packs, and maybe PD wants to give the i3 a friend.
Then there's the list of new cars or re-modeled standards:
BMW M6 - We don't have a road-going variant yet.
BMW M3 (E30) - this has been one of the most requested cars since the car/track suggestions forum was created, in contention with the Mk. IV Supra.
Bugatti Chiron - Honestly given PD's seemingly close relationship with the brand that ignited with the VGT project I'm surprised we don't have it already.
Caterham
Ferrari 488 - surprised we didn't have their latest offering in any form (N, Gr. 4, or Gr. 3)
Ford GT '17
Jaguar E-Type
Jaguar XJ220
McLaren - 720S or P1 (don't see more than one popping up at once).
Porsche Cayman/Boxter - we don't have a non-racing version yet.
Porsche - literally anything from their history from the 356 to the 918, road or racing.
Volkswagen Beetle and Scirocco (Gr. N) - we don't have road-going versions of these yet.
Volkswagen Golf Mk. I
A (or a couple/few) classic Le Mans racers to fill class with the '67 Le Mans *trilogy (I know, the Ford isn't there yet) - looking at the era, the only other manufacture producing something in contention with the race winners of the time were Alpine and Porsche - PD just gave us some fresh stuff
from Alpine, so who knows.
A whooooole lot of racing versions (real or fake) of many of the road cars that have been added over the last ~3 months (Looking at you, A110 GT4).
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If I stick with the "farewell GT6" theme with the tracks, I think we're in good contention for Deep Forest, given we've already gotten word from Kaz himself that it's in the works. It seems really fitting. It's also possible for another fan favorite real-world track to show up - people have been screaming for Spa for ages now, but Tsukuba and Laguna Seca have been some fan favorites too, and especially 'Seca has some deep roots in the franchise history.
I am shocked that we still don't have Le Mans - we've had scapes there since day one, so I'm amazed it still hasn't shown up yet. I'd been predicting it's post launch arrival since Gr. 1 started getting added to the daily race options, but alas.