I'm a Scala beginner and I honestly don't see a problem with that type signature. The parameter is the function to map and the implicit parameter the builder to return the correct collection. Clear and readable.
The whole thing's quite elegant, actually. The builder type parameters let the compiler choose the correct return type while the implicit parameter mechanism hides this extra parameter from the class user. I tried this:
Map(1 -> "a", 2 -> "b").map((t) => (t._2) -> (t._1)) // returns Map("a" -> 1, "b" -> 2)
Map(1 -> "a", 2 -> "b").map((t) => t._2) // returns List("a", "b")
That's polymorphism done right.
Now, granted, it's not a mainstream paradigm and it will scare away many. But, it will also attract many who value its expressiveness and elegance.