I would rather see car mastery completely re-envisioned as "car affinity" and provide some actual worthwhile-feeling progression. I know people don't like to see stuff locked behind progression walls, but I would love it if certain special things like rare body kits and engine swaps or other cool parts (or even liveries / design options) get unlocked after achieving some amount of miles or victories or something. This way you would actually feel some sort of investment in each of the cars instead of them feeling like anonymous baseball cards and also feel like there is something to actually work for (this feeling is almost totally absent in Forza Horizon). In addition to car-specific affinity, I would also love to see a brand affinity that has an unlock tree with special vehicles or special variants plus other things that are unlocked through a progression system. This could be really deep! Just imagine having a garage that you could decorate with special brand specific merchandise. I'll just illustrate an example of what I'm talking about
Say you start the game and you are a fan of Toyota. Maybe you start out with a 2nd Generation MR2. While progressing in the game you have access to "standard" upgrades from the start (such as generic Forza wings and wheels and such) but if you level up your SW20 affinity you could unlock a Toms body kit or convert your car to the super rare SW20 spider. Keep leveling up and you could unlock the 3S-GE BEAMS engine or a group B 3S-GTE. Max out your affinity and you could unlock a full race conversion with special interior and widebody. If you are playing online and see somebody driving a car with some of these affinity-reward parts, you would immediately recognize it as something special and that person put some effort in to get them.
Similarly, for the Brand affinity, once leveling up perhaps you could unlock some Toyota halo cars like a JGTC GT500 Supra or GT-One road car. And if they ever implement some kind of better experience for player houses, perhaps you could unlock things like banners and posters for the different car brands based on your affinity level and then allow you to invite people over (ala GTA Online) to hang out at your garage.
Perhaps my biggest gripe with FH is the total lack of feeling of progression. Open world games need that otherwise they start to feel pointless after a while.
Also, and this is a minor thing but could go a long way in establishing immersion, I really wish the cars weren't automatically cleaned when any sort of cutscene occurs. Let them stay grimey until we actively wash them! Let them feel lived-in.
I agree the progression in the games isn't great, particularly in FH4 where it was completely ****ed up, but I don't think locking cars/parts behind a grind is a good system, particularly taking into account the fact that this game has and pushes multiplayer.
Locking content away divides the community in many ways.
If parts or cars that are locked away are discovered to be OP, then that frustrates people as they will feel like they can't compete unless they do the grind too... at which point they either quit because they don't have the time, or they grind it out... That means everyone left has the same stuff and it's no longer "special" or rare if everyone basically has to do it to have a chance.
It also is a pretty negative thing for friends just playing around. I can't count how many games I've had where I try to play multiplayer with a friend who has limited time and they go "oh cool how did you get that? I wanna try it" and then when I tell them they have to grind to get the gun or car or whatever it is, they go "oh nevermind" and end up playing something else. Several of my friends stopped playing FH4 because they got sick of the time-limited cars because they didn't have the time or didn't want to spend their limited time driving some car they didn't care for at a Trial event or nonsensical championship against boring AI.
It would also limit the sharing community as well, as some shared stuff would be useless to people. Imagine the confusion in the Forza Discord and so on when people are downloading tunes that they can't actually use because they don't have whatever exclusive locked part that is in the tune. The selection of downloadable paints that actually fit the exclusive widebody kits or whatever would be pretty limited too.
Horizon is a little different as it's more of a casual thing, but I think the progression in racing games should generally be limited to saving up money for cars and actually becoming a better driver or finding ways to enjoy the actual driving more... There seems to be a bit of a rift in the racing game community on this subject though. Particularly here at GTP I see a lot more "oh I hope this game has 1000 career events to do and I need to do every one of them to unlock all this stuff so that I'm special and have neat things" than I see other places. Elsewhere I don't see a lot of people that are interested in grinding out a giant check list in a game. The weekly time exclusive stuff is bad enough, as just about every damn game out nowadays wants you to log in every day for bonuses and streaks and so on it can be tough to feel like you are actually doing stuff other than whatever the daily activity is. Add in a grind to unlock stuff that you need to be competitive in whatever division/class, and it would be easy to feel like you spend more time driving cars you don't like just to unlock stuff than you get to spend driving cars you actually do like.
Since the Horizon series isn't really exclusively a racing game, it does need something else for progression... But I'm just not sure what it is that wouldn't be some combination of tedious, annoying, and/or exclusionary. With the maps being as small as they are, they can't really keep you contained to certain biomes before letting you move on. The "story" and festival characters and dialogue are trash so more of them would be awful.
Maybe expanding on the thing from FH3 where you get to build up the festivals is a better idea? Instead of making fans just a re-skin of an XP system, make it so you actually are a bit more hands-on with the festivals. You can bring in more fans by doing races/challenges near to that festival site, or ignoring a region could make a festival become less popular. They could also add something where you can expand the festival by building more seating, building more concession stands, maybe setting up static displays and choosing what cars are parked in them... That could add more customization as well as potentially some depth with some options for region/season... like building ice cream concession stands during winter won't earn as much money/attract more people, or they could make certain kinds of booths/displays/etc add bonuses.
Something like that could allow for some customization, allow players to choose how/where they progress, and if balanced right could be something that adds some depth for those that want it, but also be kind of "set and forget" for people who aren't fussed with maxing everything out all the time.
I do like the idea for having cosmetic rewards for manufacturer affinity though, as that is just neat stuff that would let you show off your love for a brand/car and wouldn't have a negative impact on gameplay if you missed it. Banners, posters, and props for your garage would be cool. Things like old service signs, wings/hoods/parts off famous race cars that hang on the wall. I'll stick with your Toyota example, so something like a rear wing off a Toyota Eagle GTP. Maybe posters from famous movies where a manufacturer's car made an appearance, like for the 2000GT in "You Only Live Twice" but I guess that could be a licensing nightmare...
Could also make branded driving suits and helmets, like an Ivan "Ironman" Stewart set. Could also do shirts with logos including vintage logos... maybe an old '80s simple Toyota text with the yellow/orange/red stripes, maybe a TEQ logo.
Instead of doing some dumbass dance when you win a race, your character could hold a giant plastic Toyota logo, or get splashed with mud as a reference to the '80s Australian Hilux commercials, or something like that. If they stick with the quick chat things, you could unlock stuff like that "Is that a Supra?" or "I'm too sexy for the drive-through" or "BARRYYYYYYYYY."
By having them be just cosmetic, they would truly be for the people who loved them and not just something everyone has to do to get the parts/bodykits/cars unlocked.
Sheesh what a wall of text.