The way I see it, and the point I tried to make before, is that when Oda drops a hint, it's not always something obvious.
When he dropped the hint that a character would die, he anthropomorphized the ship into a character then axed it when it was convenient. Did people think it was a character before? Maybe crazy people. Up until Sky Island, it was a boat. It was better than their previous boats, and it was a nice boat, but it was a boat. If your car broke down and you had to send it to the scrap heap, that would be a shame, but you wouldn't have a funeral for it. But Oda then wrote the ship a "soul" with the whole Klaubautermann thing, all the fans said "aha!" when the ship got axed, and all the strings were tied up then.
Powerups and timeskips are generally the result of either lazy writing, or an author that is bored with the direction things are going and wants to shake things up. Sometimes it's both. Too many shonen manga serials are stuck on that whole "if the next enemy isn't tougher than the last, then the story will become stale" thing. When you're just trying to keep the attention of 15 year old boys, that's just how it goes. Don't think Oda can really turn One Piece into a full on seinen series and have it catch on, but he's got such a wide fanbase (heck, I'm old, and I'm sure there are other old farts who've read it for almost ten years like I have) that maybe it would work. I'm betting Luffy has a big 18th birthday celebration on Amazon Lily when he heals up. I just don't see Oda jacking up their ages and darkening up the plot much more, because it probably would need a fairly major artistic design shift to really work out…and I think a lot of the current fans would not like it. The whole thing is about becoming a Pirate King...that probably wouldn't be such a goal if it were more of an adult storyline. That storyline would probably focus a lot more on the political/revolutionary aspects of the series rather than attaining status. I don't think they've really got enough there to really make that all that different from much else, but maybe Oda can refine what's there.
There's just too much of a youthful, optimistic message that the storyline is built around. Thing is, with Naruto, Kishi could timeskip a few years and the show is still shonen, it's still speaking the same message and keeping the same kind of fans. With One Piece, you timeskip 10 years and keep the same theme, and you have a bunch of late twenty-somethings acting like high schoolers. That could be pretty lame.