I really like the idea of the the timeskip and this was a great way to basically end the first part of One Piece.
I agree with the other posters that when One Piece returns it will be a whole new game in terms of what to expect. The characters will all be changed to a certain degree.
-Oda has to age them of course with the youngest showing the most obvious development.
-In terms of Haki I think it won't just be Luffy who will learn to use it. I believe the characters that will develop the use of haki, and need to the most, are Zoro, Sanji, Ussop, Robin and possibly Brook. I look forward to seeing how, if at all, any of the crew, that is not Luffy, develop haki.
-The rest of the One Piece world will also be different and just how different is something I am looking forward to seeing. It is a new age after all and it will be in full swing in two years time. We'll see how many Supernovas are still in it and how far, the ones that are still going, have gotten in the new World.
-Looking forward to seeing who replaces Sengoku and how, if at all, the World Government as changed its role in pursuing pirates. (Do they move more towards the Akainu type absolute justice, or is it more of the same?)
-See how much Colby and Helmeppo have developed.
-Blackbeard is of course going to be very important in the second half so just seeing what he has done in the New World in the past 2 years will be interesting.
Plenty more things I could list but most of all I am just excited to finally see the crew back together again.
It will be a long one month, but I am sure Oda will not disappoint when One Piece returns.