data UList (a :: TYPE 'UnliftedRep) where
  UNil :: UList a
  UCons :: a -> UList a -> UList a

This would guarantee that values stored inside the list would not be thunks. It would likely live in something like GHC.List.Unlifted, since it uses GHC-specific things.

An example of something it might improve is the implementation of Control.Concurrent.QSem.QSem:

data QSem = QSem !(MVar (Int, [MVar ()], [MVar ()]))

this could instead be:

type MVarIO = MVar# RealWorld -- type synonym for brevity

data QSem = QSem (MVarIO (Int, UList (MVarIO ()), UList (MVarIO ())))

Note that in this example the tuple inside the outermost MVarIO boxes the Int - this tuple could be represented as a datatype which ensures the unboxing on the Int.

The main idea here though is that now you can use MVar# inside of the UList.