When role-playing, rolling dice is rarely about determining whether you succeed or fail. GMs generally won't let bad rolls turn into TPKs. Rather, the dice are there to determine what success costs you.
I'm imagining a system which works something like this: at the start of an encounter, add up modifiers. Then, roll a d12 on the success table. You can choose to pay the success cost or the failure cost, or, on a positive roll, you can reroll with a cumulative -1 penalty.
Success cost Failure cost 12≤ No adverse effects No adverse effects 11 Minor concession No adverse effects 10 One injury No adverse effects ... 3 Major concession No adverse effects 2 Max injuries. Minor concession. ... -3 One death Max injuries. ... -6≥ Party death One death (basic idea; no attempt made to balance)
The nature of a concession would have to do with the threat being faced. For example, against a pack of werewolves, a minor concession might mean taking a break to treat a character's bite wound; a standard concession needing to stop and search for a medicinal plant; a major concession that a character is infected with lycanthropy.
The basic mechanic would become pressing the party's luck by rerolling positive but undesirable results. This works equally well for all sorts of checks, though the table above is probably only good for combat. The players' role would be narration: how did their modifiers get applied to the situation? How was the cost was converted into success or failure?