Personally, I think that the critical thing would be to have a map where racing smaller and slower cars is actually fun. Tight roads with slow corners where even a Beetle will need to brake, rather than the previous style of making every road 6+ cars wide with even the hairpins allowing a fast and sweeping line. The FH5 rally expansion took a tiny step in that direction with the mountain pass on the west side, but it's still got very broad roads in general.
In terms of map design, twisty roads are basically directly opposed to making it drive like a NFS2 track, with casual players able to do 200mph in their hypercar everywhere. I think it's going to be a difficult balance for Playground to strike, if they do want to introduce a range of kei cars. They work in Gran Turismo because tracks like Autumn Ring Mini or Tsukuba are fun even at Wagon R speeds, but I don't know how much of a Horizon map can be dedicated to roads like that without annoying the players who just want to blast about in a Bugatti.
It's not unheard of to produce a map with tight roads either, TDU SC has a small area of Ibiza in 1:1 scale where doing 70mph through the oldest parts in an Abarth 500 starts to feel fast, and JDM has roads that are almost frustratingly narrow at times. Both are much more niche titles than the Game Pass leading Horizon series though, and the latter has a much, much lower average performance level for its car list.
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