I think if you rly study AC/DC's catalouge you'll notice a very obvious and natural sound progression, I mean listen to LTBR and then throw on Black Ice if it didnt say AC/DC on the album someone could probly convince you that there totally diffrent bands ...am I right?
and while the songwriting dosnet change that much album to album things like Brian or Bons voice have distinct tones to them that you can relate to certain albums
I mean c'mon its AC/DC they'll be slamming there full on balls out 4 chord rock and roll classics forever, theres no need to change the formula, hell its been working for decades