Man it's really true that an Author's art can drastically improve. Like If you compare the beginning chapters to later chapters of Tokyo Ghoul the art has significantly improved. The shading and details of the art is great.
So MangaGaf are there any Authors that you can name where their art has significantly improved further on a series.
Author of Shaman King improved so much as it went on. It started off as a really different style.
Edit:
Koe no Katachi 51
This chapter made me feel depressed.