If there's one thing that Shonen Mangas love to do, it's going out of their way to make better versions of everything they've introduced. Things with more power, and more features, to both obstruct and aid the protagonist.
Devil fruits are no exception - the story so far has shown us three types of devil fruits, and it has generally ramped up their power as time goes on, showing the weaker versions of a type first then showing the stronger ones (in recent memory there's been plenty of grossly strong logia's and paramecia's introduced). Where do we go from here?
If One Piece is going to go as long as most people think it will, then Oda likely has plans for a 4th type of Devil Fruit. Given that it'll be introduced as the rarest type of fruit, it may be (in part) referred to as the legendary class of fruits - limited in number (less than 10 alltogether), with powers that can encompass aspects of the 3 lesser classes.
As for what the fruits could be to where they wouldn't exactly fall into any of the existing catagories, think broad macro ideas to where it limits to fruit to only having a very limited amount. Examples of broad spectrum ideas that would be things like "Knowledge Knowledge fruit" (user gains near omnscient knowledge of the world/technology), "Power Power fruit" (user gains power/strength in a variety of different forms all at once), and so on. Essentially, fruits that would make the person who ate them turn immediately God-tier (even moreso than the strongest logias).
Of course, the above guesses are merely that - guesses, but you can be sure that at some point in the story, a 4th type of fruit will be introduced that'll at the very least have the qualities of extreme rarity, low quantity, and absurd power (moreso than logias) - it's the shonen manga way afterall.
