It would be nice if you could modify the interface to allow people to create/look for lobbies going for the deeds or achievements.
a) Deeds: the host should select a deed when creating a game. The player looking for a game - either the particular level or the quickplay - should be able to select the option to search for deed lobbies (and maybe even “I don’t care”, leading them to both deed and regular games). The deed search would never be selected as default. The deeds can still be joined only at the beginning. (Rejoining the deed should be possible, provided it was the player who started with the group and the elapsed time is less than 4 min.)
b) Achievements: the host should be able to click on an achievement button when starting a game, both qp and particular campaigns, signalling to anyone joining that the primary goal of the game is one or more of the achievements of the campaign. The game should be clearly marked as such in the lobby browser. Other players should be able to ignore the achievement games in their searches.