I have discovered that most are from previous titles, but i’ll state the new-to-Forza cars as well to be part of this list.
Returning: (including some Motorsport cars not joining Horizon yet)
1977 AMC Pacer X
2010 Abarth 500 esseesse
2013 Abarth Punto SS
2011 Alfa Romeo Giulietta Quadrifoglio Verde
2018 Alfa Romeo Stelvio Quadrifoglio
2008 Aston Martin DBS
2012 Aston Martin Vanquish
1998 Aston Martin V8 Vantage V600
2016 Aston Martin Vulcan
2010 Audi R8 5.2 FSI Quattro
2009 Audi Q7 V12 TDI
2013 Bentley Continental GT Speed
2010 Bentley Continental Supersports
2010 BMW M6 (E63)
2011 BMW Z4 sDrive35is
2000 BMW Z8
2014 BMW M235i
2012 Bowler EXR-S
2011 Cadillac CTS-V Coupe
2012 Cadillac Escalade ESV
2013 Cadillac XTS Limousine
1990 Chevrolet Camaro IROC-Z
2002 Chevrolet Camaro SS 35th Anniversary
2010 Chevrolet Cobalt SS Turbocharged/2005 Supercharged
2011 Chevrolet Spark
2014 Chevrolet SS
2011 Chevrolet Volt
2012 Chrysler 300C SRT8
2006 Chrysler Crossfire SRT6
2011 Citroen DS3 Racing
1971 De Tomaso Pantera
2013 Dodge Dart GT
2006 Dodge RAM SRT10
2005 Dodge Neon SRT4 ACR
1986 Dodge Shelby Omni GLHS
2004 Ferrari 612 Scaglietti
2012 Ferrari F12 Berlinetta
2011 Ferrari FF
1966 Ford Country Squire
2013 Ford Focus ST
2006 Ford GTX1
2013 Ford SVT F-150 Raptor Shelby
1957 Ford Thunderbird
1991 GMC Syclone
1992 GMC Typhoon
2016 Spania GTA Spano
2010 Gumpert Apollo S
2014 Honda Civic Si Coupe
1995 Honda CR-X Del Sol SiR
2018 Honda Odyssey Elite
2000 Honda Prelude Type SH
2009 HSV W427
2011 HSV GTS
1952 Hudson Hornet
2013 Hyundai Genesis Coupe 3.8 Track
2003 Hyundai Tuscani Elisa
2013 Hyundai Veloster Turbo
2003 Infiniti G35 Coupe
2012 Infiniti IPL G-Coupe (G37)
2014 Infiniti Q50 Eau Rouge
2009 Jeep Grand Cherokee SRT8
1991 Jeep Grand Wagoneer
2010 Kia Forte Koup
2018 Kia Stinger GT
2006 Koenigsegg CCX
2009 Koenigsegg CCX-R Edition
1968 Lancia Fulvia
2012 Range Rover Supercharged
2014 Range Rover Supercharged
2003 Lexus IS300
2013 Lexus GS350 F Sport
2014 Lexus IS350 F Sport
2002 Lexus SC430
2009 Lexus IS F
1962 Lincoln Continental
2017 Lincoln Continental
2005 Lotus Elise 111S
2002 Lotus Esprit V8
2011 Louts Evora S
2014 Maserati Ghibli S Q4
2004 Maserati MC12
1991 Mazda 787B #55
2009 Mazdaspeed 3
2011 Mclaren MP4-12C Coupe
2009 Mercedes-Benz ML 63 AMG
2005 Mercedes-Benz SLR Mclaren (unsure due to the issue of Mercedes and McLaren, but 2007 722 Edition might be a better option)
2009 Mercedes-Benz SL 65 AMG Black Series
1949 Mercury Coupe
1970 Mercury Cougar Eliminator
2012 MINI Cooper JCW Coupe
2018 MINI JCW Convertible
2003 Mitsubishi Eclipse GTS
2006 Mitsubishi Eclipse GT
1998 Mitsubishi FTO GP Version R
1992 Mitsubishi Galant VR-4
2010 Morgan Aero SS
2018 Morgan Aero GT
2014 Nissan Juke Nismo RS
1985 Nissan Safari Turbo AD
2007 Nissan Sentra SE-R SpecV
1969 Oldsmobile Hurst 442
2012 Pagani Huayra
2004 Peugeot 206 RC
2007 Peugeot 207 RC
2011 Peugeot 308 GTI
2010 Peugeot RCZ (or consider the 2014 RCZ-R)
1958 Plymouth Fury
1971 Plymouth GTX 426 HEMI
2000 Plymouth Prowler
2005 Pontiac Aztek (you asked for it XD)
1988 Pontiac Fiero GT
1968 Pontiac Firebird
1969 Pontiac Firebird Trans Am
2002 Pontiac Firebird Trans Am Ram Air
2009 Pontiac G8 GXP
1969 Pontiac GTO “The Judge”
2009 Pontiac Solstice GXP
1993 Porsche 928 GTS
2012 Porsche Cayenne Turbo
2009 Porsche Cayenne Turbo S
2010 Porsche Panamera Turbo
2058 Quadra Turbo-R V-TECH
723 Quartz Regalia (including the Type-D)
1990 Renault Alpine GTA Le Mans
2010 Renault Clio RS
2007 Renault Clio RS 197
2010 Renault Megane RS 250
2003 Renault Sport Clio V6
2014 Rolls Royce Wraith
2016 Rolls Royce Dawn
2010 Rossion Q1
2006 Saleen S281 Extreme
2008 Saleen S331 Supercab
2010 Saleen S5S Raptor
2005 Scion tC
2012 Scion tC
2010 SEAT Leon Cupra R
2009 SEAT Ibiza Cupra
2007 Shelby GT500
2012 Spyker Aileron
2010 Spyker C8 Laviolette LM85 (or get the regular 2001 C8 Laviolette)
2010 Subaru Legacy B4 2.5 GT
1996 Subaru SVX
2002 Suzuki Liana GLX
2011 Suzuki SX4
2014 Terradyne Gurkha LAPV
2016 Tesla Model S P90D Ludicrous
2012 Toyota FJ Cruiser
1995 Toyota MR2 GT
2011 Toyota Prius (CVT car go weeeeee)
2006 Vauxhall Astra VXR
2012 Vauxhall Astra VXR
2009 Vauxhall Corsa VXR
2010 Vauxhall Insignia VXR
2004 Vauxhall VX220 Turbo
1966 Volkswagen Double Cab Pick-up
2008 Volkswagen Touareg R50
2016 Volvo Iron Knight
2015 Volvo S60 Polestar
2017 Volvo XC90 R-Design
2016 W Motors Lykan Hypersport
2011 Wiesmann GT MF5
New-to-Forza cars:
2021 RAM TRX
2021 Toyota GR Yaris
2023 Toyota GR Corolla
2021 Toyota GR 86
2022 Subaru BRZ
2023 Nissan Z
2008 Dodge Caliber SRT4
2021 Lotus Emira
2019 Nissan Frontier PRO-4X
2022 Mercedes-AMG GT Black Series
2017 Suzuki Swift Sport
2020 Toyota Sequoia TRD Pro
2017 Infiniti Q60 S
2003 MG TF 160
2002 MG ZR 160
2013 Range Rover Evoque Coupe
1995 Mitsubishi Lancer Evolution III
2006 Volkswagen Golf R32 Mk5
2017 Volkswagen Amarok V6 TDI
2022 McLaren Artura
2023 Ford Ranger Raptor
2008 Honda Civic Type R (FD2)
2019 Honda Fit Sport
2005 Hummer H2
2008 Hummer H3
2020 Ford Explorer ST
2023 Ford Bronco Raptor (who asks for another new Bronco that one of them is already in game?)
2021 Hyundai i20 N
2022 Hyundai i30 N
2022 Hyundai Kona N
2022 Hyundai Elantra N
2022 Isuzu D-Max Arctic Trucks AT35
2009 Savage Rivale Roadyacht GTS
2020 Polestar 1
2018 Honda Ridgeline RTL-E
2021 Lynk & Co 02
2021 Acura NSX Type S
2002 Jaguar S-Type R
1997 Mitsubishi Pajero Evolution
2007 Alfa Romeo 159 TI
2013 Alfa Romeo MiTO Quadrifoglio Verde
2021 Alfa Romeo Giulia GTAm
2023 Porsche 911 GT3 RS
Sorry if this is too much for you, but those are the things I want to see in the future, say whatever you want on this 
I will likely update this post if i think of a new car.