This is an interesting problem and, while I’m not sure I can suggest a solution right now (might think about it some more and come back to you), for what it’s worth here are my thoughts (and apologies if I’ve replicated other folks’ points):
So why would I like to make this work? It would speed up turns.
That’s a very good reason. Last year I played in a HEX game where the GM periodically led us to a place where we were almost immediately ambushed by a mook swarm, followed by some higher level acolytes and then eventually the big bad named villain. It was turn-based, and followed the standard player after player attack, take some hits or inflict some damage, next player, next player, etc. This made for a very long scene, and a game where a high percentage of it was taken up by this kind of action. Fine in a three- or six-weeker, but in a four hour con-game, or even a club game where you’ve all got to leave by 10pm, not so good.
One roll per turn:
The one problem with an overall roll for the turn is that it can be too encompassing, especially when the number of enemies is pretty high.
True. The whole point of swarms is that they are supposed to tie up the PCs, a purpose that one roll per turn nullifies.
I'm thinking that normally a character gets a free defense roll for a round but they have to pay for any other rolls they make in a turn with "actions".
Yes, I like trade-offs, and paying for stuff. But, instead of trading one action (attack) for another one (defence)- which I think could also work by the way- how about paying for extra defence with increased stress or reduced health, which couldn’t be healed in-round (if you have a healer character) and would last a few turns?
Secondly, with swarms, as mentioned by J.K. in another thread, you often get the “I’m not being paid enough for this, I’m out of here” reaction by mooks, whereby if you kill a ccertain number of them the rest simply melt away. A common ad hoc way of dealing with this kind of situation, but how about solidifying it, saying that if you kill a certain percentage of them- 25% or a third, say- the rest melt away automatically?
I was talking to one of my players and he mentioned introducing skills that are used to help defend someone else. This way if a PC flubs a roll, someone else can come in and save him. It actually makes teamwork really important. I'm not entirely sure what to call a "shove someone out of the way" skill.
I like this idea a lot. In the aforementioned game of HEX, naturally we were able to help each other by throwing weapons to a character under attack, but not only did that use up our turn, we still had to roll for STR to throw the weapon, and AGI to catch it, which lengthened the round even more.
I would call this either Rapid Intervention or Spontaneous Intervention, which the helping PC could be statted and roll for, off something like Quick Wits or even Agility. You could either say for every three points of QW/AGI they would get a free point of SI, or if you have 4QW/AGI on a d10 (or 5 on a d12 etc.) you would get a free point of SI, and thereafter you could spend points on increasing it.