We all have learned that the Marines under Sakazuki have been pretty busy with conscription after the Battle of Marineford among other aggressive things. However, I can't help but wonder what Kuzan and Borsalino would do if either of them succeeded Sengoku instead. I honestly can't tell how much of an effect either of them would have on the Navy as a whole, but I am admittedly not confident that they would immediately rise up to the level of responsibility being Fleet Admiral entails. At least not at first.
Take your time in thinking this line of thought over. And please take it seriously.