Mhhh the way I'm interpreting the whole 5 year-deal is that the manga 'ONE PIECE' is going to end when Luffy finally finds the titular treasure, but it won't be the end of his story xd So perhaps Oda will continue the series, but possibly under a different title since the main mystery, after which the manga is named, has been found...?
Also by the sounds of it the big war is going to start shortly after Wano, and we already got a good reason why the Strawhats might go after the WG once the Yonko Saga is over (that being Sabo and Vivi's possible capture). So I don't think Luffy is going to reach Raftel before clashing with the Marines at least one more time xD
That epilogue theory does seem to make some sense, although I doubt it'll be only 10 chapters :d Could be that it'll be a few volumes of clearing up some final stuff: where all the side characters ended up, the Strawhats fulfilling their dreams and maybe returning home(?), what happens to the world after the Government is overthrown, and a final conflict on Luffy's part (possibly the whole giant Strawhat thingy). That could easily span another 1-2 years or so