Re: One Piece Mega Convo (Spoilers!)
Oda is so masterful a storyteller that we can't see the forest through the trees.
This arc isn't about freeing Ace, or meeting old friends and enemys.
It isn't about powering up or learning Haki.
It isn't about gaining new nakama, or reuniting with old nakama.
Some of these things will happen along the way but they aren't what it's about.
It
is about tying up loose ends.
It is the
End of an Era and the
Beginning of a new one.
End of an Era:
Whitebeard has a number of reasons to attack the World Government. Some of which are less obvious:
1) Rescue Ace.
2) Kill Blackbeard.
3) Punish the Marines for inciting this fight. This reason probably includes personally defeating Sengoku whom he would consider to be the primary offender. Would also explain why Sengoku is so stressed out right now.

4) Whitebeard is no longer in his prime. He is getting old. At the very least, he is sick and at the very worst he is dying. Either way, going down fighting like a pirate is preferred to dying in bed a sickly old man. He would rather take out all his enemies
now while he can still fight with most of his strength.
Oda has made a point of not actually killing anyone in One Piece except in flashbacks. The purpose is likely to make it much more immediate and dramatic when it does happen. The Great Pirate Age will end with the death of it's most important living member.
The Beginning of the New Era:
Depending on who defeats Whitebeard, the age will change to one of several ages. Although, like any age in this world, it wouldn't be acknowledged until you were well into it.
These are the ages for the bad guys:
The Age of Marine Supremacy
In this age, the balance has shifted towards the Marines. The yonkou are weakened or eliminated and eventually the shichibukai are eliminated as no longer needed.
The Age of Smiles
Doflamingo seems to be a merchant more than anything. We keep seeing his business ventures and his "smiley face" logo all over. He also is
Liberating Islands so maybe he is an aspiring Yonkou and plans on snapping up some islands that WB was holding.
The Dark Age of Pirate Dreams
This age would be much like the Great Pirate Age, just darker as pirates would be more willing to sacrifice their nakama to further their own ends.
It is also possible that Luffy could start an age at this time. A number of situations could crop up where Luffy ends up being the figurehead of the new age. For example, if Luffy was the only one left standing at the end of the WB vs WG battle. Or if WB survived the battle but overheard Luffy declare that he is the "future pirate king", WB would probably challenge him.