We all know the treasure One Piece! Now lets remember first of all what Oda said at the beggining:
<quote>Once upon a time there was a great pirate
His name was "Gold Roger"
He had conquered every single treasure that existed
His last words before his execution inspired people around the world</quote>
Oda isn't the kind of person to lie, if he said every single treasure that existed he might have exagerated, but that means that OP contains treasures, not only spiritual ones like FD says, but the ttile makes sense
Roger was not the kind of guy to lie either, he said treasures, his treasures he won't lie(here is the possibility that FD is right)
Then Rayleigh was asked about One Piece, he told Robin that the void century's history is part of One Piece
We know that when One Piece is found a war will start...WHY THE HELL WOULD A WAR START BECAUSE SOMEONE DID A JOURNEY! A war doesn't begin because of a journey, but a war with the WG begins when the true history is found, that doesn't include the fact that OP is a treasure, but it includes the fact that the WG would want to destroy the one who is pirate king.
Then... we saw how Roger tried to convince Newgate to get One Piece, but he didn't accept it and said he didn't want it...a journey? the title? Edward Newgate didn't accept it just because it was a title and a journey? Maybe? But we know that he had a family and that One Piece didn't matter to him that much so he might've left it for the new generation
Then at Marineford...that moment was so epic, as his last words he said: ONE PIECE IS REAL!
As a note, many thousands of pirates died because of the words of 2 men...From what we've heard Roger is allot like Luffy and like Ace...We saw that even they understand the importance of gold and money, but don't value it as much as others, but Luffy won't kill his enemies, but even more! innocent people! He always ran away from them to avoid combat. Roger would've been like that, this is subjective, but would he really send thousand of people to death and when they finally reach One Piece they'll discover there is no gold...THAT WOULD BE SERIOUSLY FUCKED UP! Imagine getting there risking your life hundreds of times to get that huge amount of money, but you find nothing...then how would you feel? You'd go mad... I know you will say, but you'll realize that your nakama are more important and the journey was great enough to fill your life...you've fulfilled you dreams, but for other people those dreams aren't the same as Luffy's, it would totally suck if there wasn't at least 1.000.000.000 there and people would suicide on the spot because of the shock
And now I'm going back to being objective: Whitebeard said that Teach isn't the one Roger is waiting there