@devilfruitgiant:
**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.
And you were doing so good, until it got to the "general" section of your theories. I highly HIGHLY doubt Flamingo would join blackbeard, as someone pointed out they have opposite beliefs about piracy. Also, Flamingo will probably be an arc boss, where as Blackbeard will become the "final boss" as you were saying at the beginning. I would love to see Usopp become as good of a fighter as Sanji/Zoro(like you said not in terms of strength)
@devilfruitgiant:
Hawkins: turned into a scarecrow, redirected damage inflicted onto others
Kidd: has control over metal (appears to be magnetism)
Law: moved things around in space
Apoo: even if you dismiss his attack as possibly being something other than a devilfruit (after all, Brook can fight with music), arm (which was working perfectly normally) transformed into a woodwind instrument. We have seen devilfruits where the users body trasforms in a similar manner (think of bellamy and Mr 1)
Bonney: Zoro commented that she looked like a kid and then a lady. Also while she was around marines their ages changed
Bege: Controlled the size of things and this was clearly seen
Drake: Truned into a dinosaur
CONCLUSION
All of those abilities are obviously devil fruits. Saying that Luffy being able to stretch is not a devil fruit ability is more plausible than suggesting any of the abovementioned supernovas don't have devilfruits.
The only supernovas without obvious devilfruits are Killer, Urouge and Zoro. Urogue demonstrated an ability to increase in size and power, but he is not the first character to perform such a feat which is why you could argue that he doesn't have a devil fruit. Still, a size increase like that is pretty remarkable, and all the other supernova captains have devilfruits so I don't think it is unreasonable to suspect he has eaten a devilfruit.
If you touch a hot iron,what do you do? You remove your hand. You don't wait for someone to tell you that the iron is hot. Similarly, if you see something that has to be a devilfruit, you know its a devilfruit before you are told what the ability is.
Oda has created a world which follows certain rules. If something looks odd (like zombies and ghosts), there is usually a devilfruit involved. If not, the phenomenon can be explained with logic that is consistant with the way the one piece world works (like franky). We have seen nothing so far, apart from devilfruits, capable of making someone turn into a living scarecrow. Sorry to disappoint you. You can find living scarecrows without devil fruits in "The Wonderful Wizard of Oz"
I'm glad you typed this because you just saved me a lot of work. I feel the exact same.