Yeah, also. There's the coolness factor and other mechanical interaction. So coolness factor isn't enough for a lot of players, I know, but it's one that I stand by as enough justification for almost anything. So, let's ignore it, as "we" always do
1) Other mechanical interaction could be one or more talents, for instance reducing the penalty. This of course makes TWC quickly contingent upon gaining those talents, if the talent(s) changes the mechanical working of the this action, that is to say if it directly manipulates the dice pool. We, or I at least, don't want TWC to be an automatic go-to for max damage output, yet I don't want it to be pointless either. If going for a penalty reducing/removing talent, I'd probably opt for requiring separate successes for increasing the damage of each weapon, and limit TWC weapons to light weapons, no talent to allow normal one-handed weapons.
2) If I stay with a flat non-adjustable -3 penalty, I do believe additional damage should apply to both weapons. I'd still consider limiting TWC to light weapons only if 1 success adds 1 damage to two different weapons.
3) An overlap between mechanical benefit and coolness would be allowing new weapons to be used, that is to say non-light, non-heavy, one-handed weapons, which I think may be a better solution than allowing penalty reduction (but that's me,) yet it still represents power creep. It increases damage output, but the if we retain the penalty, damage potential is kept somewhat limited. I wouldn't combine this with penalty reduction. Regarding increasing damage, I'd probably do 1 success for both weapons, as long as the penalty was a flat -3.
All of these should be play-tested though to make sure what works best in practice (which I assume will vary between groups,) as opposed to armchair game theorising
4) To hit a second target could either be a talent: you declare before you roll and use the modifiers coming from the target that is the hardest to hit, this talent allows you to spend the success for activating the second weapon to also hit a second target. Or just add a stunt that requires an additional success to hit a second target (so 3 total successes: 1 to hit, 1 activate second weapon and 1 hit second target.)
5) Regarding Morph: I'd probably not let you hit with a third weapon, sure you can do an extra 1 AP action per turn, which could translate to a quick attack, I know, but using the ideas I have here I wouldn't let a third or fourth arm let you wield and
use more weapons for attacking simultaneously. Arguably you could use the third arm to make a quick attack first (or afterwards,) and make a normal TWC attack too.
I've moved into the lands of TWC being a talent in its own right I think, if not its own little mini-game... Or at best a series of linked talents. Fun exercise, but its getting a bit much now
it's moving away from what I understand the design intent of the system seems to be.
After some thought, I think I'd just go for TWC being a combination of a quick attack/shot and a normal attack/shot. Not sure I'd add the extra AP for an additional attack as a default thing, I'm not sure its needed, but it could arguably be a Gunslinger talent that let's you add 1 extra attack-only AP when attacking with a light weapon once per round, this attack-AP must be with used with a different weapon. Sure, this is more rolling of dice, but this seems to be more in line with the design intent... perhaps.
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