Fairy Tail is too derivative of One Piece to be the next One Piece. Fairy Tail is pretty much a watered-down version of One Piece. Similar characters, themes, execution, etc, but without really memorable or strong execution.
So Natsu might do some some cool stuff, but that'll only be the shadow of something GAR that Luffy did. The Fairy Tail guild might emphasize friendship and "nakama", but the members are too many and some a bit too detached to achieve that closely-knit relationship the Mugiwara pirates have. Grey had a sad past where he lost his beloved teacher, but it's nowhere near as BAWWWW-inducing as, say, Nico Robin's past.