Since people have been resurrecting old topics and what-not, I thought that the Crazy Theories thread was one that deserved to be brought back.
I'll start it off with a reprint of my All Blue Theory as I've added a bit to it since I originally posted it.
Now, assuming that there is indeed an All Blue, for there to be fish from all four Blues, the most likely location would be the intersection of these seas. There are two possibilities; one is the entrance of the Grandline at Reverse Mountain while the other is where the Grandline cuts through the Redline.
I doubt it's the latter; the channel that cuts the Redline at Mariejoa could easily be mapped and thoroughly explored. The Redline is so massive that it should appear as a dim line on the horizon to the north and the south no matter where you are in the channel. A person should be able to get their bearing as a result without being dependent on a log pose. There should also be constant traffic in, out, and around Mariejoa as well. If All Blue was there, then it should have been found.
No, the more likely answer is that it is at the other intersection of the four oceans.
My theory is this; the ocean current that makes up the Grandline is made up of four currents, each of which runs through a Blue. Some of these are cold water currents and some are warm water depending upon the climate of the Blue that they run through, which is why we have so many different weather patterns on the Grandline as that particular current is warm in spots, cold in others, and varying shades of each in others.
The Grandline divides into five currents at Mariejoa, the most powerful of which continues on the final island while the other four head into a Blue, cross back to Reverse Mountain and form the Grandline again.
Now, Reverse Mountain is shaped like a four armed star underneath the water with a sharp knifelike protusion sticking into each Blue. The top side of the blade forces part of the current up the mountain while the bottom forces another section of the current downwards, which drags fish from each blue along with it. Voila, All Blue. The main current of the Grandline slides under the mountain and rejoins the the other four currents on its trip to Mariejoa.
Now, All Blue is hidden inside of the mostly hollow Reverse Mountain, which explains why it's largely a legend. The only people that could possibly have seen it for a fact would have been on Roger's crew.
This also explains why the One Piece has not been found, as it's located inside of Reverse Mountain. Perhaps the only way to access it is by a series of caves; some of the caves are inhabited by local monsters, possibly local inhabitants, various traps set by Roger, or even some of Roger's crew who were assigned to protect the One Piece until he returns. The caves may even not be accessible except at certain points in a certain fashion, like rowboats at low tide.
The only way to find Raftel is to know which cave will lead you to the entrance to All Blue, which may be one of two ways; by being a D, you have some specific attribute that tells you which one is correct or it may require the ability to read Poneglyphs. It's possible that there might even be multiple paths to the One Piece, but that only the strongest can survive the trip.