In the world of One Piece, the Devil Fruits are an extremely valuable resource, responsible for giving Luffy his super-stretching powers as well as beefing up more than a few of the Straw Hats' foes over the years. There's at least one variety of Devil Fruit, however, that has never appeared in the series, and probably never will.
The Devil Fruits are a mysterious resource in the One Piece universe, and they can be found around the world, despite their extreme rarity. Those who eat a Devil Fruit will gain some kind of special power--it could be something truly amazing, like time travel, or less impressive, like transforming into an axolotl. Such power comes with a downside, though: anyone who's eaten a Devil Fruit will lose their ability to swim and deal with water, which can be a pretty big problem since the fruits are primarily found at sea. Additionally, anyone who's consumed one Devil Fruit can never eat another, as this always leads to death.
Devil Fruit are lumped into three major categories, but there are individual, recognizable fruits among them that make it possible to predict what power they might give. Because information on the Devil Fruits is often kept secret by their users, no one seems to possess a full list of all available fruits, meaning many types won't be known until they're eaten. Dozens of varieties have been encountered so far, and in question and answer segments, One Piece creator Eiichiro Oda has spoken of a few varieties which haven't appeared in the series directly. One of these, mentioned in Volume 35, is called the "Ero Ero no Mi," most directly translated to "Love Love Fruit" or "Erotic Devil Fruit."
Oda mentions the concept of the Ero Ero no Mi in connection to a question about whether he wishes Luffy had eaten a different fruit and gained another power. He says no, as he spent a lot of time considering this before the start of the series, but then mentions the Ero Ero no Mi as an idea he's continually come back to (over a dozen times). Oda gives no indication as to what this fruit might do, but judging by the typical Devil Fruit naming conventions, it's likely that it would offer something like an irresistible attraction or incredible beauty. Obviously, such an ability would radically change the story if it were given to Luffy, but it's interesting that this idea has never been fully explored for an enemy or ally, despite coming to mind so many times. It's likely that whatever ability the Ero Ero no Mi would confer, it's not terribly appropriate for a shonen series aimed at young boys.
Oda's question and answer segments are generally considered canon by fans, as he typically uses them to explain concepts or clarify details. Whether that means there's an Ero Ero no Mi floating around somewhere in One Piece's high seas waiting to be eaten is left up in the air, but fans shouldn't hold their breath hoping to see it.