Ok, so I'll finally come out with my predictions, which hopefully are all based on at least some form of trend or reasoning that I believe could be going on in the heads of those at PD who decide which cars to release.
One noticeable theme was the Ferrari supercars. They basically completed the lot with GTO, F40, F50, Enzo & LaFerrari (and even the 250GTO as the granddaddy of them all). I think we can expect them to add the other premium models from GT6 at some point, but the ones they already released (512BB, 330) were well chosen according to their significance. In my opinion, the next ones in terms of significance would be the 365GTB Daytona and the 250 Passo corto. I'm curious, if they will bring some newer machinery like a 488, 812 or Portofino, but I think it's less likely.
Most likely next car: 365GTB Daytona
Which brings me directly to Porsche. From the currently available cars it seems to me that they are seeing the GT3 as the prime 911-variant (probably because it's the purest driving machine), so they are trying to complete its heritage. After 991, 997 & 996, it's a bit less obvious which models would best fit that lineage. In my opinion a 930 Turbo or a Carrera RS 2.7 would fit the description. A logical step for me would also be the equivalent of Ferrari's top tier. In Porsche terms that would be the 959, Carrera GT and the 918 Spyder. (Those would be top of my wishlist, but I dare not hope we will get them soon)
In terms of race cars, the most obvious would be either 917, 956 or 962.
Most likely next car: Carrera RS 2.7
I feel Mercedes is currently one of the more underused brands, so my bet is the 300SL Gullwing will reappear quite soon. The GT series featured the then-current versions of the "SL" several times, but currently the SL has become even more of a luxury car, so it doesn't really fit into the game that well. The SLR McLaren may make a comeback at some point.
Most likely next car: 300SL
Regarding BMW, there's only 2 more M3's to add now. One is not that iconic (E36), but I would expect the E46 to appear one day, hopefully in its CSL-variant. I tend to think though, that we'll see the 507 again much sooner.
Most likely next car: 507
It's also pretty likely from what we've seen, that they are working on completing the Corvette-history. So, C1, C4, C5 & C6 are still missing. Anyone's guess which one is next, but there were premium models for the C5 & C6 already. Most likely next car: C6 ZR-1
Some more Lamborghini cars are also pretty likely, so I would expect the Murcielago, Gallardo and also the second Countach to arrive eventually.
Most likely next car: Murcielago SV
Other anthologies to be completed or expanded upon:
MX-5, Type R, Fairlady, Kei cars, Skyline
Most likely next cars: MX-5(NA), Civic Type R(EK), Fairlady(Z34), Suzuki Cappuccino
Other cars that are likely to return because of their historical significance and because there already was a premium model in GT6: Lexus LFA, Pagani Huayra, Lancia Stratos, Renault R8 Gordini, Honda S2000, Camaro Z28 '69
That much for returning cars. New cars though are anyone's guess really, but I would deem there's a chance we will some day see some of the following:
Toyota Supra '19 (because PD was involved with the Racing Concept's launch already)
Aston Martin Vantage '18 (just a hunch)
Bugatti Gr.3 (this is an oddity, as Bugatti is the only brand with a Gr.4, but no Gr.3 car; but would it be a Veyron, Chiron...??)
And finally here's some iconic cars that ought to be included at some point in the game, but who would know what PD thinks in that regard? So this is obviously not based off of anything except for the fact that they have included a surprisingly large number of cars that would make it on to some top 10-list of cars from each decade, but somehow these have not made it into the game yet...
Aston Martin DB5, BMW M1, Audi quattro/Sport quattro (and yes, I know there was a standard quattro)
I'll pack up my tea leaves now...