the way i see it:
WB–> impel down to rescue ace --> trap --> whitebeard overcomes the obstacle --> BB is called for help --> BB kills WB, but all prisoners are free -->
BB is sent to recapture all of them --> WG starts to suspect BB is out to become pirate king -->they send an admiral to check it --> fight in new world, BB wins, stays in new world. Later he will meet ruffy, smoker + aokiji as his enemies.
SH --> Fishman Island, nobody tells ruffy about ace, cause nobody knows -->fishman arc at the same time WB raids ID -->they reemerge in the NW --> underwater, so no news about what´s happening on the "surface" --> when they reappear, there are evil pirates everywhere --> new age of pirates as said by flamingo.
maybe flamingo had the plan of freeing all the prisoners from impel down before whitebeard and they meet in the way.
what better way to start a new era than to release all prisoners of the old era at once? only concern i have with my theory is that WB actually has to die there, otherwise no new era could begin. ( remember Gol D. Roger´s death and the impact it had on pirates everywhere? i think it will be the same if WB, the currently "strongest man", would die.
and it MIGHT actually have been flamingo´s goal for a long time to kill WB next to freeing all of ID. i guess he has kind of a backstabbing personality, perhaps the possibilty of killing WB will be offered to him while the ID raid takes place.
enough silly thinking for the next 3 hours.