Since we got two new cars in this update that fill out (or duplicate if you're feeling negative) existing model lines, and it's a noticeable trend that they've been filling in a bunch of these lines during a lot of recent updates, I thought it'd be interested to see the progress that has been made towards completing them.
I'm only counting OEM road cars, and am also not counting PD tuned variants like the Gr.X Road Cars.
Audi
TT
Currently in the game: 2003, 2009, 2014
DLC Count: 1 (2009 TTS)
Notes: Line is probably complete.
BMW
M3
Currently in the game: E30, E30 Evo, E36, E46, E92
DLC Count: 1 (E36)
Notes: We don't have an F80, but we do have the M4 coupé equivalent and I don't know that saloons are what PD are going for with their roster construction. Line is complete otherwise.
Chevrolet
Corvette
Currently in the game: C1, C2, C3 (Stingray and Convertible), C4 ZR1, C6 ZR1, C7 (Stingray and ZR1), C8
DLC Count: 2 (C1, C8)
Notes: The C5 is the obvious missing piece to make this line complete. They could also add the ZR1 C8 once that's out as they do like adding the zippy versions of modern Chevrolets (the Camaro and C7 both have a base and hot versions).
Dodge
Viper
Currently in the game: SR, ZB, VX
DLC Count: 0
Notes: This line is likely complete forever unless they want to add a convertible version.
Honda
Civic
Currently in the game: EG, EK ('97 and '98), FK2, FK8, FL5
DLC Count: 2 (EG and FL5)
Notes: The question here is does anyone, PD, Honda or userbase, care enough about the 2000s Civics? They're there if PD want the line to be complete, and the FD is even a Premium, but I think they've added the correct missing Civics so far. There's also pre-'90s Civics, but those feel quite unlikely given current car update trends.
Mazda
MX-5/Roadster
Currently in the game: NA, 2015 ND (S), 2022 ND (NR-A)
DLC Count: 1 (2022 NR-A)
Notes: Missing the NB and NC. The NB would be some people's choice for the most obvious "Gran Turismo" car that's not yet in the game, you'd have to think it's coming eventually. There's also a hardtop ND version, the RF, that feels very PD-like to add.
RX
Currently in the game: FC RX-7, FD RX-7, RX-8 Spirit R, RX-Vision
DLC Count: 0
Notes: I could see a pre-facelift RX-8 being added, but I get why it'd be a bit of a long shot. There's also the FB RX-7 out there, but that's not been in the series since GT2. My general rule is that if an '80s/'90s JDM car didn't get a variant into GT4, it's probably low on PD's priority list. If not either of those, I think this line is probably complete until they release an RX-9.
Mercedes/AMG
GT
Currently in the game: GT S, GT R, GT Black Series
DLC Count: 0
Notes: There's a new generation of these out. The GT3 car isn't based off of it yet thought, I wouldn't expect the road car until after Merc update their GT3 again.
Mitsubishi
Lancer Evo
Currently in the game: III GSR, IV GSR, V GSR, VI GSR Tommi Makinen Edition, VIII MR GSR, IX MR GSR, Final Edition
DLC Count: 3 (III, VIII, IX)
Notes: Missing the I, II, and VII. All are less popular variants, and Mitsubishi themselves don't like the VII much because it was terrible in WRC (no game currently licenses it). I also wouldn't be shocked to see a standard VI GSR, or a non-Final late 2000s X.
Nissan
Silvia
Currently in the game: 2 S13s, 2 S14s, 1 S15, a 180SX and the Sileighty
DLC Count: 2 (both S14s)
Notes: The Silvia still has a long pre-S13 history, but one that PD has never really cared all that much about (only two pre-S13 Silvias in GT2 or GT4 when there were a billion trim specs of S13 and S14). I think they're done here for the forseeable. Maybe an outside shot at the 270R now that the 400R has re-established Nismo as a brand?
Skyline/GT-R
(yes, I know not all Skylines are GT-Rs and vice versa, I'm still lumping them together)
Currently in: KPGC10, KPGC110, R31 GTS-R, R32 GT-R Nismo, R32 GT-R V-spec, R33 GT-R V-spec, 400R, R34 GT-R V-spec Nur, R35 GT-R, R35 GT-R Nismo, R35 GT-R T-spec
DLC Count: 5 (KPGC110, R31, R32 Nismo, 400R, R35 T-spec)
Notes: If you search "R34" in this forum, you will find like ten posts of mine in various prediction threads going "every other modern GT-R has a Nismo variant, so the R34 should get one too!". I'm sticking to that line, that's one of the three gaps I expect them to fill eventually, along with an R30 road car and a pre-facelift R35 (since that's a really important car to PD's history). They could also reach back for a '60s Skyline, but I really don't think PD are that interested in pre-1970 Japanese cars right now, the classic focus is definitely on Europe and (especially) America.
Z
Currently in the game: 432, HS30, Z32, Z33, Z34, the new Z (I don't know its model code)
DLC Count: 0
Notes: Maybe they add an early '80s Z31 at some point? But other than that I suspect they may be done for a while here, it's pretty complete.
It's also worth noting that every Nissan road car in the game is in one of those three lines, there's not much diversity there.
Porsche
911
Currently in the game: 901, 930 Turbo, 964, 993 RS, 993 RS CS, 996, 997, 991, 992
DLC Count: 2 (901, 992)
Notes: There's one for each generation now, but there's so many variants (Turbos, Carreras) that I don't want to say this is done. Especially since Porsche are still fairly new to the series, there's still novelty there if they decide to add an extra vehicle or two to a generation.
Subaru
BRZ
Currently in the game: 2015 S, 2018 STI Sport, 2021 S
DLC Count: 1 (2021)
Notes: Basically, take any new OEM 86 we get, add a few months, and the BRZ version will likely come.
Impreza/WRX
Currently in the game: 22B STi, Ver. VI, Blobeye, 2014 WRX
DLC Count: 1 (Blobeye)
Notes: There's a few that could come here. Wagon versions of the first generation, a Bugeye variant of the second generation, any third generation car (one is the GT Auto animation car, let's not forget), and the second generation WRX all feel like they could come into the game and make sense.
Toyota
86
Currently in the game: 2015, 2016 Limited, GRMN, 2021
DLC Count: 0
Notes: I think there's nothing new they'll add currently - gen 1 is pretty well covered, and there aren't that many gen 2 variants. But if you said that I had to predict one car that eventually comes to Gran Turismo, that guess would be "whatever the next generation Toyota 86 is".
Supra
Currently in the game: A70, A80, 2 GR Supras; also the FT-1 if you want to count it
DLC Count: 0
Notes: Similar to the 86, I think they've squeezed all of the juice that's currently there, but the moment there's something new I imagine PD will be on it quickly.