Okay , so you want a system that ' s openended that is , chance of success or failure can asymptotically approach 0 but never reach it without using exploding dice . Successcounting pool systems are the usual way to do this . i rarely like rolling a single die for test results , the bell curve helps in terms of bad or good luck as it averages out on a middle result , but it feels like its hard to make use of in other than the dice bonus vs threshold mechanic . Do you want degrees of success and failure ? Your other points don ' t include them . If you don ' t , the die result isn ' t meaningful in that sense . There are only two outcomes , success and failure , and you can ' t have a centerbiased distribution of only two things . There is a significant difference in how centerbiased vs . flat distributions work , but it ' s not that . It ' s in how they ' re affected by modifiers .