At the end of the series, Sisko is "with the prophets." I don't think that means he has biologically become a prophet, but living with them and being entrusted as their "emissary," he must have the ability to intervene in the time in creative ways. Given that the prophets were able to return Akorem to his time without disturbing the course of events or changing knowledge of the past, I should think Sisko could do all sorts of crazy things. Make sure the Bell riots unfold as they should? Prevent Jennifer from dying? Prevent Jadzia from dying, perhaps while retaining Ezri Dax? Any other possibilities?