@Masta:
It would be possible to end it in the next 200+ chapters. Here me out:
-There are several arcs which are extremely small and short in length, yet they answer lots of questions and cover lots of material. A PERFECT example is Roguetown. It explained that people world-wide are suffering from the world govt., proved that there are a very small majority in the Navy who have clean hearts (Tashigi, Smoker), it let us know some villains may be returning with vengance (Buggy), AND it foreshadowed conections between Luffy, Roger, and the other 'D's and that they all may die the same way–laughing. And thats not even the only "tiny" arc that covered a crap-load of material: The same goes for Morgan (Zoro's past and Coby's departure), Romance Dawn, etc...
I think the only problem with this logic is that Logue Town didn't really answer much, and I don't really agree that a lot happened. It introduced characters like Smoker/Tashigi, reintroduced characters like Buggy/Alvida, and gave Zoro his 2 new swords. And foreshadowing the connection between Luffy and Roger, was just that. Foreshadowing. To me, it really only opened up more questions (particularly, Dragon, how Tashigi/Zoro will evolve, Smoker's obsession w/ Luffy), and it's a lot easier to pose questions than to answer them. Trying to prove a math theorem can be taken as an extreme example of this. The statement of the problem can be done in 1 line, but can often take pages upon pages of derivations to prove or disprove.
For the other short "arcs" you mention I think a lot of why they were so short is that there weren't a lot of characters yet. Romance Dawn was basically about Luffy and Shanks, Morgan was mostly Luffy, Zoro, Coby, Helmeppo, and a little Morgan. Also, each fight took like 2 pages, as opposed to 2+ chapters.
@Masta:
-All of the things we have left behind/unanswered (Shanks, Whitebeard, Blackbeard, Buggy, Dragon, True history, Raftel) are ALL connected in one way or another and it is most likely they will be explained/resolved within the same or close-connected story arc. Seriously, do you think theres is going to be an Alabasta 2.0 just about the relationship between Shanks and Whitebeard, and who Dragon is???
For your second point, saying all those things are closely connected is just conjecture. None of them have really been addressed in detail. I can give you Shanks-Whitebeard-Blackbeard, since it was said in the manga itself. I can even say maybe Dragon-True History, and True History-Raftel are related, since Dragon seems to be a pretty big player, and Robin's remarks in Skypeia. But it's hard to say the two different groups are related.
I agree with you that it could take just a few chapters to answer a lot of the questions…IF Oda was just trying to answer the questions. But Oda is trying to tell a story (naturally flowing, proper introduction, good build up of tension, interesting climax, good resolution), and in order for it to be interesting, and for the answers to come out as a natural part of the story, I think it will take significantly longer.
I admit, there is a bit of thruthiness here, but please forgive me.
Just as curiosity, how do you feel the rest of the story will pan out? Here's my guess:
end of Enies Lobby ~20 more chaps.
Arriving at Red Line (including some mini arc suggesting they're getting closer) ~20 ch.
Crossing Red Line (and they'll encount some problem) ~20 ch.
Conflict with Doflamingo ~ 40 ch.
Approaching Fishman Island + some conflict with Jinbei ~ 30 ch.
Finding 3 more crewmembers along the way ~100 ch. total (this might be an underestimate, considering the whole Long Ring-Current is really about losing 3 crewmembers [Robin, Usopp, Merry], and gaining 4 [Robin, Usopp, shipwright, ship], a net of +1, while hinting at the true history)
BTW, this is already 200+ ch.
Stuff happens on Elbaf ~ 20 ch.
Conflict with Blackbeard ~ 30 ch. Probably, Ace will show up here, but I doubt Whitebeard will yet.
Stuff happens with the other 3 Shichibukai (Bear, 2 unknown) ~ 100 ch. total
Conflict with Whitebeard ~ 40 ch.
Dragon/Revolution ~ 70 ch. (if they get involved. significantly less if not)
Reuniting with Shanks ~ 10 ch.
Resolution of the SH's dreams ~ 10-20 ch
Post One Piece, after Robin reads the Rio Poneglyph? ~ xx
Total: 520 ch.
For the ones that seem kind of long, I included the mini arcs that lead up to the main one. Consider that this number does not include arcs that come out of nowhere, like the whole Skypeia arc. This is just counting the ones that I can kind of assume are sure to show up.
Whew…That was long.