And not just data formatting.
It would be nice to be able to define a data type and some rules and limits of progression.
I could see the value in defining an arbitrary data type that was comprised of a fixed set ("Apples", "Pears", "Oranges", "Bananas") with no progression (ie, no set member has precedence or rank) or perhaps some with progression or rank (fetus, infant, toddler, child, adolescent, adult, senior). Cells formatted as belonging to a data type would only accept those values as valid entries, and sorting would apply the set's rules of simple progression if there were any.
It might help for other numeric-based data types, such as IP addresses, where it would be helpful to define rules of progression around some kind of delimiter. If they could only add one new data type, I wish it was IP addresses.
There's probably complex ways of doing this with macro/scripting, but, they end up being complex and one of the main reasons so many people use Excel because it makes it trivial to manage lists. Trivial tasks that get made complex end up being done sloppy.