This is a two part question. Please excuse any mistakes I make as I am not an expert on 19th century Germany.
1. How could Bismark have remained in control of Germany? From what I know, he held most of the political power in the German Empire under the reign of Wilhelm I and Frederick III, but when Wilhelm II became Emporer, their disagreements became more and more pronounced which culminated in Bismark's resignation. What is the best POD to keep him in power? Could Germany get an Emporer who agreed with Bismark's policies (through certain people dying or not dying at the right times)? Could a POD in Wilhelm II's personal life make him more concilliatory to Bismark? How long could Bismark have remained as Chancellor? Could he have stayed on until his death in 1898?
2. What effect would such a scenario have on history? Would WW1 go better for Germany if Bismark had remained longer? Could WW1 have been avoided altogether? (This is probably unlikely, but who knows?)