Okay, here's how I think things will go:
1.BB takes Ace(or his dead body) and turns him to the WG. He gets the Shichibukai position and goes on with his plan.
2. Shortly after BB gets made Shichibukai the news reach WB and Shanks. WB is ready to rush and save Ace(or take revenge), but Shanks advises him to not rush things. WB doesn't really listen to him and starts calling up his fleets. Shanks sees WB can't be talked to and tries to reach Luffy so he doesn't do anything stupid.
3. Dragon hears about the news, and also about WB getting ready for a battle. He starts making a plan on what he will do next, and if the time is right to attack.
4. Luffy and his crew are just on Merman island doing whatever they are doing, they don't know about the news jet.
5. If Ace is dead Garp storms to Sengoku and they start arguing about BB and him getting the Shichibukai position. If Ace is alive he goes to Impel down to talk to him.
6. WB gets his fleets ready. Dragon decides the time is right to fight. Shanks is still tryng to reach Luffy somehow, but than decides he should stay near to the action and then decide what to do next.
7. Luffy and his crew just came from Merman island, they somehow find out what happened to Ace. If Ace is dead, Luffy wants to be alone for a while, we see alot of flashback of his time with Ace. Then he decides to storm the WG. If Ace is alive they storm the WG right away to save him.
8. An epic battle starts, we see alot of some side characters fighting, and of course Luffy who goes all badass. We see great sea battles, and characters falling to the ground one by one.
Thats pretty much it