yup, i mean sure he can make one about rikudou/ashura/indra/kaguya and etc.. but thats like ancient history. And won't really matter much since the world was reformed after the Warring states period.
However, i guess it'll help explain the genetics or something, for those who like genetics knowledge.
This would allow us to see how hashirama died, How powerful tobirama was when Hiruzen's sensei, Hiruzen in his prime (during multiple wars), and the upcoming of the sannin and their developement. You also get to see Onoki in his prime.
Hiruzen Gaiden includes everyone automatically, so he would be the best person to do one for.