Didn't a time skip occur right at the beginning of OP? Chapter 1, Page 48
I think a time skip will occur and that it will be 3 years, making Luffy 20. Now the reason I think this will be the case is something cultural about Japan. 20 is considered the age of adulthood over there. If consider the story of OP as a coming of age story at its core; then you could consider the 4 other oceans as the childhood years, the first stage of the grand line as adolescence, and the final stage of the grand line as adulthood. One could consider the Red Line as a literal wall of adulthood, one that the SHs are simply not ready to cross yet. Adding to this idea you can compare the setup of this world to that of the Japanese school system and society. At primary school you normally dont choose what school you goto, much like there is only one path for each of the Blues to get to the grand line. At the end of primary school and onto middle school you get to choose what school you attend and generally start down your path of life. The beginning of the grand line is similar to this in that you choose your path from 5 magnetic fields. As you become an adult your finally part of society with the privileges and responsibilities associated with that. All students need to become adults, at which point they choose which path in life they are going to take. This is like the end of the first half of grand line, and they emerge from the other end of fishman island into the new world, the world of adulthood, Where they will choose they will choose a new path.
If you look at how the story as progressed up until SA arc then you can see that the SHs have gone up against some challenging situations, but nothing has been completely over the top that the were unable to defeat their opponents. However, once they reached the red line, the wall of adulthood, they were completely defeated to the point where Luffy tells everyone to escape and save themselves.
Chapter 513, Page 7
When Silvers Rayleigh was told the SHs that even if he told them the truth about the void century they wouldn't be able to a thing about it. Chapter 507, Page 06
All the opponents during and after the SA arc minus the events that occur on Amazon lilly are way beyond the power of Luffy. He couldn't defeat Sentomaru, let alone the Admirals on SA, at Impel Down he didn't completely defeat Magellan, and at Marineford he couldn't even save Ace. In all these situations he was saved by someone who was much stronger than he or his crew were.
So whats gonna happen next? Well I definitely think some powering up is going to happen for the rest of the SH crew. In the next immediate chapters I think were going to see how strong of a bond there was between Luffy and Ace, this will make the loss all the more hard for Luffy. Think the greif will be so hard for him that he will loose sight of he dream for a little while. The geography of where he is right now will also play apart in it. It's a physically isolated place and I believe Hancock still retains her Shichibukai status so the rules about Amazon Lilly being off limits are still in effect, the perfect place to hide while the heat dies down. Also since Amazon Lilly imbues Haki into their weapons and Boa can also use it, it would be the perfect time for Luffy to master his own Haki.
That leaves one loose end, the Thousand Sunny, its still at SA and I don't think it can remain there its too close to Mariejois and the WG. I'm sure the marines are looking for it right now. So it may be possible that Rayleigh, Hatchi, and maybe some others might sail the ship away and possibly go out searching for the missing crew. Since everyone has a piece of Rayleigh's Vivre Card he can easily go out and gather the rest of the crew. At the end of the 3 years they'll have gathered all the missing crew members and meet back up with Luffy, perhaps persuading him to come back and return to becoming Pirate King. Then they sail off to the red line (making a pit stop at SA to drop off Rayleigh and Hatchi) fully powered up and ready to kick some ass in the New World.