Ever since the introduction of the Festival Playlist, the Horizon series has been almost entirely relying on it with introducing singleplayer content. While this may seem not inherently wrong, there are many issues that come with such system - all the championships are time limited, meaning not everyone may be able to experience some of them; there are no permanent championships, only individual races that can be completed once and then forgotten; and finally it removes a lot of depth from the singleplayer portion of the game because it ultimately limits players from making their own adventures.
I believe the Horizon series needs a new and permanent list of events, similar to older Motorsport titles. The ‘Horizon Events List’ would consist of themed championships that allow players to obtain expensive, rare and exclusive cars outside of the Festival Playlist. They may be longer and more difficult to reflect the value of the reward more accurately. Forza Motorsport 2 should be taken as inspiration - early game events rewarded players with decently powered street cars and some rare classics, while later championships unlocked more performance cars, super car legends, and then finally actual race cars; the length of championships increased in every consecutive league.
Furthermore, this Events List should also allow players to complete in both Co-Op and PVP modes, and there should be specific Multiplayer hoppers for all championships included.