It's never to early to start a wishlist for the next Horizon game. These games are (or should be at least) on long development cycles, and I wouldn't be surprised if they already have some staff doing preliminary game design and prototyping.
In terms of what I want to see in FH5, in a nutshell I want to see them borrow the best ideas from previous Forza titles and other car game franchises like TDU and GT, and fix many of the longstanding issues with the franchise in terms of the awful UI and exclusive car systems, do away with the Games as a Service mentality, and separate singleplayer and multiplayer into different game modes like it was in FH1.
Anyways, here's my wishlist:
Map:
Much larger map (at least the size of one of the TDU2 islands)
More realistic map layout (roads leading off the map so it actually feels like a real location, not a self-contained pocket dimension)
Lots of interactive locations around the map (car dealers, car meets, service stations, player houses, festival sites, etc)
Racing:
Driving school (optional driving tutorial that teaches players how to drive, starting with basics up to advanced race driving and drifting techniques)
Multiple realistic race tracks scattered around the map like in TDU2 (mix of dirt and paved circuits)
More interesting race restrictions like in FH1 with route-appropriate vehicles instead of every race just allowing every single type of car with the AI auto matching (still leave the open car choice available for players to make custom events)
Cross country route and jump intensity should be greatly reduced, add stadium events instead (with monster trucks!)
Car collecting:
Car dealers. Instead of just a generic autoshow, cars could be purchased from various dealers around the map. New cars would be in the appropriate lot, and older cars could be purchased from a used car dealer. There would also be a "buy online" option which would function similarly to the current autoshow for players who don't want to use the dealers and view/test-drive the cars.
Customizable garage. Player would have an upgradeable garage (eventually warehouse) where they can organize their cars and show off their collection.
Progressive car unlocking. As players progress through the game, they unlock new cars for purchase. At the beginning, only common cars are available through dealers and the used car lot, but new dealers and classic used car lots would unlock over time. In multiplayer, cars are organized into more arbitrary tiers (like in FM7) and are unlocked through earning MP experience points. Cars unlocked in singleplayer would be unlocked in the multiplayer autoshow (where they could then be purchased with multiplayer credits), but not vice-versa.
No time-limited car exclusivity. Rare cars "drop" in the used car dealer periodically, can also be bought and sold by players in a global AH. Extremely rare RL cars (like the Skyline GTLM) could be earned via in-game challenges, and would be permanently added to the garage and couldn't be removed/sold so they can't be lost (like one-time reward cars in FH3/FH4 can if you accidentally auction them).
Vehicle features:
Ability to toggle vehicle features like start/stop engine in freeroam, turn headlights on/off, put windows up/down, use blinkers, retract sunroof/put down convertible top (if available), etc
Add motorcycles to the game
Vehicle customization/tuning:
Improved vehicle customization with the option to change interior colors (plus manufacturer colors for interiors) and more appearance upgrades like NFS (more spoilers, change headlight color, etc)
Improved paint interface
Player storefronts for paints/tunes (with the ability to test-drive tunes before buying and Forzavista your car before buying the paint)
Improved vehicle tuning with ECU programming for tweaking the powerband and shift points, plus the ability to reduce PI by downrating the engine with the ECU and also adding weight.
More tire options (non-rally snow tires, low tier dirt/all terrain, rain, etc)
Ability to test-drive vehicle with proposed upgrades and tuning before buying
Driving model:
Properly simulate modern dual clutch automatic transmissions and launch control systems
Properly simulate active aero on modern performance vehicles
Better ABS/TCS/STM simulation (modern RL systems work much better than they do in the game)
Rebalance PI model to fix issues like we're seeing with the bone shaker and daytona coupe being OP for their class
Bring fuel usage over from the Motorsport games, and add service stations around the map
More realistic "simulation" damage model. The current model is not worthy of the name "simulation", could still be left in as "arcade" or something, but I want to see a NFS Shift style damage model where hitting a wall at high speed means bits and pieces of car flying everywhere and immediately puts someone out of the race.
AI:
Improve so it's more consistent and feels better to race against (I know that AI coding is extremely difficult, but I would like to see something that at least doesn't blatantly cheat at the start and rocket to the front for 90% of the race before abruptly slowing down to allow the player to pass).
Offroad:
Do basic materials modeling and impose a stiff penalty based on ground clearance, tire compound and drive type for driving offroad. Driving a RWD sports car into soft sand should get the car stuck immediately, with a button to rescue the car back to the road. Driving cross country will get road cars stuck rather quickly. Don't even think about crossing rivers without appropriate ground clearance and a snorkel.
Driving assists:
Change the driving assist model to remove the arbitrary bonuses for shifting modes. Cars should come with whatever transmission and driver assists they would realistically have. Modern automatics with the ability to select a gear could toggle between auto and manual shifting while driving, and cars with defeats for the ABS/TCS/STM could also have these toggled. Vehicles without this ability in RL could gain the ability in game with an appropriate upgrade. For older cars that are more difficult to drive, assists could still be enabled where the drivatar helps the player out a bit by shifting, pumping the brakes, keeping the car stable, etc, but not as efficiently as a skilled player who has put the effort in to learn how to drive (i.e. the drivatar's shifts will be rather slow like the automatic is now).
Avatar:
Character creator for our avatar with the ability to customize appearance and change outfits (do NOT put clothes in wheelspins, put them in a clothing shop and maybe have special reward outfits for completing certain events)
Ability to have the avatar get out of the car in freeroam and in special map locations (dealers, car meets, festival sites, player houses, etc) and interact with NPCs and potentially other players or just walk around and enjoy the scenery and take photos.
Wheelspins:
Gone. Maybe have a small gambling minigame on the side where you can pay a few credits and see if you get a reward, but make it a small side thing, not the main form of progression.
Story:
Have a main story with a couple racing paths to complete it. Maybe something like in FH1 or NFS where you're working your way up the ranks to be the best racer (a bit stale, but we haven't had a good NFS game in a long time so it might be ok to bring this plotline back), but at least give the player some in-game reason why they're racing. There could be multiple paths to the end depending on which race scene the player enjoys the most instead of just one linear path.
Have the street scene play a bigger part, including police chases. Rewards could be unlocking rare cars and special abilities (call a street scene buddy to help out in freeroam or during a race)
Crazy-taxi style randomized events (not a scripted story) for alternative ways to earn credits between races
Anyways, just a few (well, more than a few) thoughts on what I would want to see in the next Horizon game.
2007 Toyota Blade Master G
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