For B&B's core mechanic, I wanted to more-or-less use Blades', but have it use a mix of polyhedrals; for one of those polyhedrals to be a d20; and for rolling a 20 on that die to be something meaningful. What I came up with is this;
When making a threat roll (here using terminology from Deep Cuts), you roll a d20 and two Trait dice; traits cover the same conceptual space as Action Ratings and Attributes, except they're rated on a dice scale, from d4 to d12. Some effects can have you add dice, or adjust the sizes of the dice used. Then, you pick the d20 and any two of the other dice you rolled and count the number of dice which rolled 4 or higher; a roll of 20 on the d20 counts for two. 4 is a critical, 3 is a success, 2 is peril, and 1 or fewer is a threat result (again, using the Deep Cuts terminology).
This does have some downsides in relation to Blades' core mechanic - notably, additional threats is a lot less elegant - but I'm a fan of platonic solids and am otherwise pretty happy with it.
