Pocket healer behind your back
I was thinking of an imaginary "dangerous" score of an individual player, outside factors like teammates or having a pocket healer aren't part of it.
I suppose you could try something different, and add a fourth factor for "teamwork", but that's going to be complicated by the fact it depends on the other players, so you have to take their abilites into account. You'd have to know their skill, their ability to work as a team, the level of teamwork (couple of mates on at the same time, or a select four-man composition with voice comms?): horrible to work out.
Would be interesting if you could determine a "force multiplier" for working as a coordinated team with tank, heals and dps though. How much more effective would it make you than the same four as random strangers? Twice, so like eight randoms?