It would be nice if we could choose only singles or only 6-person fireteams as an option for matchmaking. I've been on both sides of that type of match and it rarely feels balanced and is often over quickly.
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IDk about being able to select off it, but Matchmakign SHOULD prioritize party size second after connections. If a group goes in with a group fo 6, they should at least being playing 2 groups of 3 each on nearly every match if the playerbase is there to make it happen.