Me and my friends likes to fight against one another when we are alone in a premade group (outside missions ofc), and it usually get one of us kicked from the server for excessive team killing.
I would love to be able to start a FFA fight that is not time or kill based within our premade group, that also would not yield any rewards or punishments, it would just be for pure fun.
It could be a toggle option for the group leader and anyone in the group who presses K will join. And typing /abandonmission will take you out of it.
If you "abandon mission" you would still be in the group and anyone who is still left in the FFA will be seen as if they were in a mission. If you decide to press K again you will rejoin.