Here, "later work" means related work done at a later date, maybe several weeks or months after.
"which elements of our argument have been most fruitful for later work" = "which ideas have been the most helpful for additional work we did later"
I hope that helped!
May 7, 2019
I don't know the exact meaning for that word, but the meaning for whole sentence is nothing but the result of that element is useful for next upcoming process( I think so) but this what the meaning of that sentece