Going back to DavidM's remark that artists don't make two important landmarks.
There are a couple of exceptions I can think of just off the top of my head.
a) Norman Cook (from the House Martins) - a very popular group in their time, who totally changed genre & name to become Fatboy Slim.
b) Damon Albarn - from Blur - again hugely successful and then setting a new precedent when he created the Gorillaz
Of course it depends what you mean by 'landmark' it could relate to popular success, critical acclaim, whatever really.
Yes I know they're not dead.
yet. (loads his sniper rifle)
ooh that reminds me, John Lennon is another example. The stuff he did after the Beatles was highly controversial and set a new precedent in musical experimentation