but these aren't plots but rather anime fillers
What do you mean with anime filler? :'( These are all in the manga, right?
Anyway, the era of Hashirama and Madara is much more interesting and dramatic than that of Naruto. At that time everybody were fighting each other, and propably there were epic dramas and battles. Theresmany things that can be done in the plot, and many characters to be explored at that time. Like Mito, the older brothers of Madara, Oonoki, the shinobi mentality at that time, and so on ...
Hashirama, Madara, Izuma, Mikoto in my opinion, doesn't have such a definite personality, it would be nice flow their characters. And characters like Kushina, Obito, Kakashi and Rin are more interesting and charismatic than their respective counterparts, Sakura, Sasuke etc... And would be interesting follow the third shinobi war in deph.
Not showing these characters and episodes is not a plot hole. But If anything in Naruto were done now, I think these respective stories has more potential than the lifes of Sarada, Boruto, and their family dramas.
So that's why spin offs is the best idea, and preferably the author not being Kishimoto, since he is sick of Naruto, and would just write shit. It's not impossible, others have already made spin offs with different mangakas, and some were even better than the original series. See Lost canvas, of saint seiya, for example.
The idea is long prequels in similar pattern, but of course, but not so long than Naruto. Just complementary stories.