343i should make custom games joinable via matchmaking.
Custom games are an essential part to Halo multiplayer, they allow the creativity that games like COD and Battlefield do not grant the player, my wish is that it should be easier for players to host/find lobbies and give us more max player games.
In Halo: Reach / Halo 4, it was easy to look at the roster and perhaps see some friends playing a map with a full lobby, except as the game gets older less people are playing and hence these lobbies become harder to find.
I am aware that online communities exist to advertise lobbies so that people can join, But it's still a good amount of effort to create a new post and have enough people view and join the games.
A proposed system would simply act like the server browser in games such as the Battlefield series, The results would show the lobbies with the most amount of players first, and show the gametype and map names.
If 343i do not already have this planned, I would love to see it implemented in a future update.