There’s a lib for that!
http://hackage.haskell.org/packages/archive/OneTuple/0.2.1/doc/html/Data-Tuple-OneTuple.html
Actually, we have a OneTuple we use all the time. It’s called Identity, and is now used as the base of standard pure monads in the new mtl:
http://hackage.haskell.org/packages/archive/transformers/0.2.2.0/doc/html/Data-Functor-Identity.html
And it has an important use! By virtue of providing a type constructor of kind * -> *, it can be made an instance (a trival one, granted, though not the most trivial) of Monad, Functor, etc., which lets us use it as a base for transformer stacks.