I think that after wano, luffy will receive the news about Sabo and he will lauch another crusade to save his brother in a paralell to the paramount war saga, but this time he will have his WHOLE crew with him to help him.
okay but like. here's what i think - sabo's situation? its not going to involve luffy at all.
the news will be released. newspapers will flutter down from the sky, and the strawhats will conveniently leave wano right before they're able to read it. the audience will see the front page - sabo captured, killed, enslaved, whatever - and the strawhats will go on their merry way, because this upcoming war is not a war for pirates, but for revolutionaries. for marines who can't take akainu anymore. for citizens who want out of the governments power.
coby - our next fleet admiral - is there. garp is there. vivi - queen and ruler and strawhat - is there. the revolutionaries are there. mariejeoise - you know, where recent chapters have hinted at is something more - is the setting. we have all these players ready for a revolution to take place, and enough stakes and characters we care about for this setting to distract for a bit!
i think the reverie....2! isn't going to parallel marineford at all, because sabo isn't going to die - there has to be new stakes, and what better stakes than the fate of the entire world! its a different playing field, a different arc, because the purpose is entirely different from marineford - to give motivation for the straw hats to get stronger, to forge onward, and to answer the question usopp posed at the end of thriller bark - what happens if we go up against an enemy we can't beat? that luffy can't beat? the strawhats are strong now, and luffys about to defeat kaido. he's near unbeatable now, as he should be at this point in the story. the point has been made and a direct parallel to marineford would be somewhat redundant. the reverie is here to shake things up - to tie up all the plot points that have pointed at a corrupt government and the horridness of absolute justice, and the dreams of coby and sabo and the rest of them.
marineford was a war and a tragedy and the beginning of a new pirate era, the reverie is going to be a revoltuon and a victory and the beginninng of a new era for the world.
tl;dr - saving sabo (if he even needs to be saved) isn't going to be luffy's fight but garp's, and vivi's and cobys, because all the signs point to the fact that a revolution is coming and the reverie is the perfect place - additionally the purpose that marineford fulfilled is over, and having luffy save sabo with his crew (tho therapeutic and wonderful and would make me cry) would be a bit redundant.
also garp is gonnna die thats my bet. he's dying i know it.