Stopping the so-called "Dark Ages" would have meant a continuation of the Roman Empire, which, despite my Romaphilia, would not be a good thing for humanity.Stopping the dark ages from happening would've meant much.
Stopping the original Semite groups from creating Judaism, meaning no Christianity and Islam.
Telling the men at the Academy of Fine Arts Vienna to accept Hitler.
Finding Gavrilo Princip and keeping him busy so he wouldn't assassinate Franz Ferdinand.
Islam probably would have come about in some form even without Judaism. Mohammed wasn't a Jew or a Christian before he developed the religion.
This might have worked, but then again, Hitler's style was so different from what was popular those days he may actually have gone more nuts at his lack of success.
Probably a good idea, at least for stopping WWI.