So, you have been warned about potential spoilers twice now, but I will still remind you that this is just a speculation and may be proven completely pointless. So, here we go.
This is something that instantly sprung to mind when I had read chapter 818. We are told that (spoilers) Raftel is difficult to find, since you would need all four Road Poneglyphs, decipher the coordinates written on them and then draw an X using those points. X would mark "the spot". I really liked this idea, thought it was a very nice way to give some indication towards the journeys end and at the same time show that there are still many obstacles to overcome...
However, you in fact (given that these are our only rules at the time) only need three out of four.
Decipher three out of four locations and then sail in a triangular course from one to the next and you would sail at some point to Raftel. Having a person whose vivre card you possess at each location would circumvent the chaotic weather.
If you had difficulty understanding what I meant ( I am bad at explaining things) draw four points on a paper and then a line from each point to each other. The result is a square with an x, or four triangles with one common point in the middle. There are six total lines, each representing a single straight route to sail, and two of them cut through Raftel. Even with two poneglyphs you would have a 1 in 3 change to find Raftel, but with three you will find it.
Now, all the possible ways to around this that I could think of;
1.Luffy refuses/is confused, and wants to do it "properly"
2.One or more points are moving like Zou
3.Raftel is invisible, sunken or in the sky, you would be forced to preform a rite, dive, or wait for a knock-up stream.
4. Every poneglyph contains more info than just the one coordinate
5. The basic rules of geometries are different to our own in the world of One Piece(would not surprise me)
So, what do you guys think? Did I miss something, did you come up with something?