Raftel is the last island in the Grand line.
Only Roger and his crew confirmed its existence, and probably one piece is somewhere in Raftel, but we know that in the new world, many very powerful pirates such as the Yonkos are sailing, why no one of them is able to find it?

I think that the only way to find Raftel is by following the lead of the Poneglyphs made by the Ancient Kingdom.
Roger found Raftel by following the passage of the Poneglyphs around the world, he said:

”I have come here and will lead this passage to the farthest ends of the world.”

So we may say that the Ancient kingdom that ruled the world during the Void Century is still alive (or only a part of it), this Kingdom fought against the world government 800 years ago but lost, so they left to the world three main keys to lunch a counter attack against the World Government.

First: the Poneglyphs that will play the role of the eternal pose to Raftel where they are hiding, by contacting them the world will know the past of the void century.
Second: three big weapons of mass destruction able to destroy the World Government!
Third: It's another Theory but I may say that, it is the One Piece.

Buzz this guys for  more interesting Theories.