**NEW THEORIES? (edit)
Main Plot** The world government will be overturned in Luffy's lifetime (after all, he's not just a Gol D Roger clone is he?)
Blackbeard will be the final villain, however Luffy will also meet Coby "at the summit" (possibly having a battle afterwards)
Blackbeard have a hand in preventing Ace from dying in the near future. He will mess up the Ace execution plan and maybe take out whitebeard as well. (Killing two birds with one stone seems like his style)
Usopp will become basically as strong as zoro, sanji, etc (in terms of fighting ability, not physical strength)
Chopper will learn to control monster point (Using similar abilities as shown by Kumadori and Lucci)
We will see another human-human fruit at some stage like human-human fruit model: giant for instance (This may explain Vegapunk's intelligence?)
General
Franky will have a temporary power upgrade in the form of cola syrup and will also upgrade his own functions at some point.
Doflamingo will join Blackbeard’s crew and immediately set off into the new world
a) He's got the height (everyone in blackbeards crew is really tall)
b) He seems to get on pretty well with Blackbeard (think of the shichibukai eating scene in the manga when they were the only ones enjoying themselves)
c) He's always talking about the "New Age" (why introduce a really powerful character with some of the best quotes in the series if he's not going to be a major character)
d) Possible awesome fight match-up with Robin
e) We haven’t seen any of his crewmates and his underlings (like Bellamy) are way too weak
f) He doesn’t seem to have a problem with leaving major things behind on a whim (think of the slave auctioning place)
Magellan will leave his post and pursue Luffy into the 'new world', teaming up with the likes of smoker
When they reunite, all the Strawhats will arrive at their ship simultaneously (sporting new weapons and attire in some cases) by different means of transport and it will be the most kick-ass moment in one piece to date. Luffy will say something like "set sail to the new world"
General
Aokiji is the strongest admiral
a) He keeps a lower profile than the other two (talks less, does more)
b) His abilities are on such a large scale (While Kizaru is destroying whole trees, aokiji is freezing whole seas. Similarly, Akaino seemed to be trying pretty hard when he destroyed that chunk of ice)
All of the supernovas will make it into the 'new world'
Hancock is only in love with Luffy because she is more prone to falling in love than other people as a consequence of the devil fruit she ate. Some devil fruits seem to alter personalities and habits (think of how magellan likes to eat poison or how Wapol is always hungry and is gluttonous in general). They will not end up as a couple but she will join the crew.