The following are quotes from Oda, either from interviews, JUMP Festas, SBS's or any other place Oda speaks about One Piece outside of the manga. Included with the quotes are the most credible sources for them that I am able to find. I am not giving my own interpretation of the quotes, everyone is free to interpret them as they wish.
They're gonna make a former boss character their partner and head to the New World.
Sources: Greg; Day 2 of JUMP Festa 2009; NUMEROUS blogs
Oda says he's the closest to a human in the series with everyone else being super human. He feels that as Usopp slowly gets stronger, his readers can feel the same way and get stronger too. Tanaka loves how when Luffy and team go off leaving Nami and Usopp behind then they both say, "Counting on you -_-;" Oda says there are certain things he wants to keep the same because it will just upset the balance. No matter who joins, he always wants Usopp to be the weakest,
Sources: One Piece 10th Treasures interview; Greg paraphrasing the interview
And this is a manga for boys so I won’t draw anything about relationships. I get a lot of letters from female readers asking me to cover some kind of relationship but if that’s what you want to read, go read a girl’s manga, that’s not my job. I don’t care if someone falls for Luffy, but it’s something that would be over before you know it. But knowing Luffy, I don’t think that kind of thing would be a problem. (laughs)
They won't fly! (laugh) Because if they fly, they won't need ships anymore. Also, they'd be able to do anything… I'm really focusing on the aspect of sailing the seas.
Source: COMICKERS interview, Roy Watanabe (interview here, http://www.apricot.com/~fuu/paradise3/oda/cm1098.html)
Also, I had a sorcerer in a short story once, but I won't be using that in the series. Any preposterous situations are all created by the devil fruits. That one point is rather supernatural, but everything else is normal.
Source: Exact same as the flying one
"TEN OR MORE MEMBERS"
Small corner brings back Luffy's declarationabout crew members. They suggest this means he'll get, "AT LEAST,"one new member in the New World. Note that in the same blurb, they count the current the number at NINE
Sources: Greg; Data Book Green
NOTE: Data Book Green (as well as any other data book) has Oda's name on the cover, but is not written by Oda. It is written by assistants and editors with Oda's approval.
Please note if you believe there are other important quotes that should be here or that there are better sources you believe I should use.
I purposely did not include Oda saying that Strongworld is Luffy's last adventure as a 17 year old because it should be obvious to everyone by now that Luffy is no longer 17.