This is a total brainstorm-level idea from a relative BB noob, in reply to Doubleskulls' comment that attrition levels might need to be equalized somewhat. I agree with this idea. The thing about bash vs finesse is that finesse takes it both ways: they take many more armour breaks, AND the casualty table gets rolled on more often because of MB, resulting in more permanent damage (which in my mind is the real issue behind why certain teams don't get played). It's a double-whammy they can't easily reciprocate without a string of doubles skill rolls.
What if MB was changed such that the +1 to Injury option was changed to a 7=KO, sort of a reverse Thick Skull. (If the victim had Thick Skull already, the two effects would cancel.) This would still remove the player from the pitch, but would equalize the Cas chance back to 1/6. Note that bash teams would still tend to roll on Injury more often since finesse teams' armour is that much less, and more hits tend to be delivered by bash teams.
And/or, a new Agility skill could be introduced that replaces the lost MB effect, something like 'Shiv' or 'Low Blow': +1 to all Injury rolls. This would increase the chance to roll on the Cas table, thus increasing the frequency of permanent injury. It would also make finesse teams more hurty toward each other -- which somehow seems an acceptable side effect.
With both of these changes in place, bash could still select Shiv/Low Blow on a doubles (and it would work in tandem with MB, with the end result more powerful than currently), but it's one more skill to get to achieve maximum killitude, thus pushing out other more situational/opponent-specific skills.
Obviously this is a nerf to bash and a boost to finesse. Politically it would probably be necessary to nerf finesse or buff bash, but I'll leave that to better minds than mine.
As you can tell I'm big on preserving the fluff as much as possible, and from a fluff perspective I don't think Piling On is defensible. And it's not socially acceptable either -- in the 2 TT leagues I'm in, it's frowned upon, but if you do take it, you take it on your killer and you only have one such player on your team. One idea I had for "new" PO was to change it to an assisting skill, like Guard. For every PO player assisting a block in no tackle zones, the hitter gets +1 to the armour roll. If this is seen as too powerful, it could be useable only if the PO player went prone, or if they spend their action to do it.
Some crazy ideas, FWTW.
