One thing that GM (and the other North American car companies) has forgotten when building 'full size' sedans -- adequate head room for both front AND rear seat passengers.
If you have access to specs for late '70's and early '80's GM full-size sedans (Chev Impala/Caprice, Olds 88/98, Buick LeSabre/Electra) you'll note that there was 40-41" of headroom in both the front & rear seats. Since the seats back then were super-soft non-supportive foam, you probably had even more actual space when you sat on them.
In recent years, the headroom figures for GM's 'full size' sedans have shrunk by 3-4" to around 37-38" for front seat and 36-37" for rear seat on the Lucerne, Impala, and even the DTS! As the support of the seats has greatly improved, you don't sink into them nearly as much as 30 years ago.
I don't think that the North American population has shrunk by the 3-4 inches which have been eliminated from these full-size automobiles' headroom. There are people out here who want to get out of SUV's and into well-engineered modern full-size cars--we just can't fit into the current models offered.
Please, please GM -- if you're going to make a modern RWD full-size sedan, give us back the North American headroom we need