There are plenty of era's in the Naruto series which have a whole host of powerful ninja and their interesting stories. Which one would you like to see as a spin-off or prequel/sequel to the Naruto series.
1. Kaguya the Alien princess: we still don't really know much about Kaguya, her race and why she wanted to make an army of zetsu in the 1st place. Did Kaguya (and her race) have the byakugan and other kekkei genkai before she ate the fruit? Did she fear other aliens and wanted to build a zetsu army for war?
2. Hagoromo and the birth of ninjutsu: I feel like it would be very interesting to see the events which started the cycle which is the naruto series. Seeing how people went from their warfare into escalating power using ninjutsu. It would also be interesting to see how samurai and other fighting groups (knights, pirates, etc) would have existed during this time as samurai existed before ninja in real life but in the narutoverse, ninja arts are so dominant.
3. Hashirama/Madara the warring states: This seemed like one of the most violent times in the narutoverse's history. This could exhibat the curse of hatred throughout several different generations and explore how clans interacted with each other.
4. Ninja war 1: I really want to see how Hashirama died, this is something that needs to be answered at some point.
5. Ninja war 2: This is an opportunity to see more kage from the time period as well as other characters such as Hanzo, Sakumo Hatake, the Sannin who were at there full power during this era of warfare.
6. Ninja war 3: This seemed to be the darkest of the ninja wars as all the villages were struggling for firepower and were forced to have children participate in war, where many lost their lives. We could explore more of the akatsuki's actions before they started their tailed beast hunt.
I would personally like to see the third ninja war, as we could see more of the jonin (Kakashi, Might Guy, Kurenai, Asuma) as they were younger in the war. Akatsuki back story is always interesting to watch since most of them are immortals or outstanding prodigies. Parents of Naruto, Sasuke and others would also be interesting as we have yet to explore some of their abilities even though most of them were clan heads and held very high positions within the village.
Edit: Showing the 3rd ninja war from Mei Terumi's perspective would have to be an interesting story. A girl with multiple kekkei genkai who lives in the "bloody mist" which kills people for having even 1 kekkei genkai. This could create a freshness in perspective as we have only viewed this world from Konoha's eyes so far.