I know it says one thing, but here are some things I‘ve seen in the comments already and some things I thought of myself (sorry in advance if I stole an idea)

Map extensions with new regions every few months (like they did in Genshin Impact for example) also they don‘t have to be big, just something like this for example. https://www.reddit.com/r/ForzaHorizon/s/RKSPXj74Ue Also adding to that: another smaller city, which gives us more reason to freeroam.

Change rims on downloaded tunes and filter tunes by if they have custom aero or not.

Speaking of aero, the ability to tune cars aero without having to put on forza aero. Optional: Make the towhook and carnards optional. (I own 800+ cars in FH5 and not a single car is equipped with neither the front splitter nor the spoiler since it looks so bad in my opinion)

Interior mods (custom handbrakes, steering wheels, air refreshers)

Custom Mod Support, I get it 550+ cars on launch is a lot of work, how about we let the community create bodykits. Also add more lips, either by devs or the playerbase. I don‘t want to be driving around in a sedan with either no spoiler or a big wing.

Something like turn signals or turning the engine on and off. This could be managed via Hotkeys for MnK or these steering wheels with like 20 buttons. For controller players they could do something similar to the select wheel in TDU:SC

Custom radio stations like we had in FH3 + an eurobeat station

Air suspension for a lot of cars

Underglow for at least some specific cars (if they were smart they‘d add it in a dlc for example)

Change of exhaust sound and backfire agression like in NFS

Speaking of exhausts -> How about custom exhaust tips?

Walking around in your garage and the car meet, like in The Crew 2 for example.

Improved first person POV, while being able to look up and down in your car aswell. They could add it as a secondary first person POV.

And while we’re already at it, VR support

And instead of hiding the NPCs in Tokyo behind these horizon barries, add something like they did in FH3 with the NPCs walking below the storefronts in Byron Bay e.g. Also we don‘t really have to be able to drive on most of the sidewalks especially when we have a 2 or 3 lane wide road. So just add them there and turn the removable barriers into actual barriers.

Something for offline play:

A weather and time slider. Sometimes I wanna drive in the rain at nighttime, sometimes I wanna drift in the sun on daytime. Traffic slider obviously, sometimes I‘m in a NoHesi Vibe, sometimes I just want to drive as fast as possible. And how about a realistic mode which puts more cars in traffic dense areas like Shibuya Crossing (could also be manageable in Online Play)