I would be interested in seeing a graph showing the number of mass concentrations as a function of their size. It looks, from a brief, purely superficial, visual inspection of the map, that the distribution is bimodal, that is, the size of the lumps that make up the moon appear to cluster around two size ranges: a Mare Crisium size of several hundred km, and a large crater size (< 100 km). If this is indeed the case, it might gives us clues as to how the moon formed.

Your question of what is below the "peel" is a good one, if the mascon size distribution changes substantially with depth, it could give us a very good picture of just how the moon formed, how long it took, and how the accretion process varied over time. On earth, the greater heat and liquefaction in the mantle would have erased all evidence of the earth's early history.

Now we need missions to other large moons in the solar system. Is there as much variation in satellite internal structure as we see in satellite surfaces? A set of GRAIL-type missions could tell us just how solar systems form.

In fact, you could launch a number of gravity missions like this, all over the solar system. I guess you could call it 'wholly Grail'.