Just as a clarification, the proposals are to add multi-dimensional slices (dynamically sized arrays). This statement is splitting hairs, but a Matrix is a container for numeric types that can do things like multiply and have a cholesky decomposition. A multi-dimensional slice is just a container for whatever you want. Such a container is very useful for matrices, but a package would still have to turn a multi-dimensional slice into a Matrix in order to add methods like Add.