I prefer pure Co-op and less Pvp, unless it is against each other, but u will need loads of B20 players.
From everything i played so far, i enjoyed "Liberation" the most. Liberation is maybe a bit overkill since the map is so huge and there is so much to do. If u want to go smaller, there are also missions done by for example "R34P3R" in the workshop like "Till the end" or "A long way". These are 4 player coops that take about an hour to complete.
Off coarse, if we have some talented mission makers, u can always make some custom missions your selves. Or use Zeus to have a game master but i never used that. Note to those that want to make their own missions, always give the enemy units waypoints and battle stance like fire at will or seek and destroy and use triggers. Dont just pop them down on the map and hope for a good mission. Also needs some testing and balancing to really enjoy a good mission.
If things kick off here, i would have a go at creating a custom B20 mission but it has been a while and i will need some time to get to grips again with the editor.