I will list the pros/cons I see below:

Pros:
- This datatype is commonly defined throughout many packages throughout Hackage. It would be good for it to have a central location

- The inductive definition of 'Nat' is useful for correctness (e.g. safeHead :: Vec a (Succ n) -> a; safeHead (Cons a as) = a;)

- -XDependentHaskell is likely to bring this into base anyway

- I believe that it might be possible to eliminate a Peano Nat at some stage during/after typechecking. I'm not well-versed in GHC implementation, but something along the lines of possibly converting an inductive Nat to a GMP Integer using some sort of specialisation (Succ -> +1)? Another interesting, related approach (and this is a very top-level view, and perhaps not totally sensical) would be something like a function 'f', that given a data structure and a number system, outputs the representation of that data structure in that number system (Nat is isomorphic to List (), so f : List () -> Binary -> BinaryListRep)

Cons:

- -XDependentHaskell will most likely obviate any benefit brought about by type families defined in base that directly involve Nat

- Looking at base, I'm not sure where this would go. Having it in its own module seems a tad strange.

I am open to criticism concerning the usefulness of the idea, or if anyone sees a Pro(s)/Con(s) that I am missing.