The Planetary Thread and Theory shouldn't get buried!
The Six Moons actualy explaines a lot:
the extreme wheather paterns… compases not working (in the GL)... Different climate and weather islands.. why Enes Lobby is allways day, why The Florian Triangle is always dark and has no stars (one of those moons is above it could be under a permanent eclipse). Oda will use (Planetary Structure and) The Six Moons to explain the whole geography of One Piece, the phenomenons, the crazy weather, calm belt, the Sky People and some parts of the Void Century.
That can all begin with Monet saying something like this to Luffy "If I don't follow my orders and complete my missions then my dream will never come true"
She loves her books, she may be an astronomer, all females in the crew have rather (our-worldly/) real aspirations (Navigator, Archeologist) and now hopefully Astronomer. It takes only one crew member to convince Luffy to go somewhere
Some people might complain saying they can't take a huge detour just to go to a/the moon/s but they can... all it would take is to go there is a DREAM. Luffy wants to help his nakama fulfill their dreams. If Robin finds out there is something that links to her dream and finding out the Void Century then she will want to go and if Monet does join (and is an Astronomer) then that is even more of a reason to go.
Plus we will get to see Enel. I believe that most the enemies/people the SH's met in Paradise, they will see again. Enel was used to show it is easy to go into space and to introduce a broader history of the Sky People (one that we might need to know about) and his Cover Story was the preview/snippet to the Moon Saga. We will see him again as we did not see him in the 'From The Decks of The World' cover story.
The Moon Saga will be used to fully explain the world of One Piece, something that needs a broader explanation (in geographical, astronomical and historical aspects). Nami could use being in space to help her draw her map of the world (help complete her dream), Robin could learn a bit more of history (the rest of the history of the sky people and maybe something on the Void Century) and Monet could complete her dream (if it is to go to space)...
STRAW HATS TO THE MOON
--- Update From New Post Merge ---
If Sabo's alive, then Ace is to, Oda never killed anybody, but since it was in a flashback, he's dead.
The Straw Hats have killed plenty of people (Not main Villains or Crews/Groups) but they have "inadvertently killed"
Like the fodder agents of CP9 at Ennies Lobby (they just flung them into the abyss, inadvertently killing them")