Forza Horizon 1 (and FM4) both had server browsers for custom lobbies that were set to public, these were an enjoyable alternative to the main official playlists. It allowed people to play just the maps and game modes they were interested in (for example, playing just Infected rather than the official playlist which alternates between Infected and King). It also allowed for increased time limits, which was needed for the large scale “Bunker glitch“ games of Infected I mention here.
These were removed from Forza in Motorsport 5 and subsequently in Horizon 2, I remember being pretty disappointed at the time, and public custom lobbies seem to be a mostly forgotten concept by now, but they’re still one of my favourite ways to play the games that include them.