Yesterday I did two Swift d12 playtest sessions with Manuel Sambs, one for Saga of the Goblin Horde, and the other for Manuel's Neon City Nights setting. We identified a few areas that need some work, but the one I'd like to talk about right now is "complications".
The Action Deck in Savage Worlds often treats clubs as complications, and that adds an unpredictable element of risk to rules like Chases and Dramatic Tasks, mechanics that I frequently use in my adventures. However Swift d12 doesn't use cards, and without complications it always feels like something's been lost when I convert the adventures. So after some consideration, I've come up with a generic complication mechanic.
Whenever the GM calls for an ability check, they may also declare a complication. The player should then roll two complication dice at the same time as their action dice.
Complication dice are d6s, they do not explode, and they cannot be rerolled with Karma Points. If the complication dice roll a double value, then the complication is triggered; the player must then make another ability check with a penalty equal to the number on the complication dice. Failure results in some sort of mechanical or narrative drawback, which the GM should describe before the player rolls.
Example: Big Brak is knee deep in the Northern River, fighting a minotaur, and the GM announces that his next attack has a complication due to the strong current. Big Brak rolls his Melee check and succeeds, killing the minotaur, but the complication dice roll double 4. The GM announces that Big Brak must now make a Muscle check with a -4 penalty (because of the double 4); on a failure he'll be swept away by the river.
I think this captures the unpredictable feel of complications, without the need for cards. It's also a unified mechanic that can be applied to any ability check at the GM's discretion, rather than a "special case" rule that applies to specific subsystems, and I think that should make it more flexible and intuitive to use.