I think it was somewhere after the Drum Island arc that they saw a lot of steam in one spot. Most members were alarmed but Nami explained that it's just a hot spot with an underwater volcano under it. She says that a new island will be formed at that spot.
Now if you pay attention and look back on early chapters, Oda likes to give subtle foreshadowing hints about new adventures.
Now the volcano may signify an island with a volcano, but this is ONE PIECE.
IMO, the underwater volcano signifies a DF user. A volcano fruit.
So what? Volcanoes are fearsome and powerful. The user will be terrifyingly powerful.
Who's most likely to possess this fruit? I'd say AkaInu. If the 3 admirals are around equal strength, then they would most likely possess extremely powerful abilities, such as logia DFs.
What do you think?