Well that starting a new series is nearly impossible, as Kishi already used his all plots . he may be clear out background stories of several char in Naruto . Like Clans that we only heard of , or previous kages.
Kishi could easily start up a new series. There is literally tons of things he could still do. However he would still struggle to try to top or equal fights. He would have trouble to find a antagonist that gives naruto and co the same amount of trouble as the villians of the war have given naruto.
The only 2 villians I can think of that would work, is orochimaru, who possessed madara, or ate shinju fruit, or perhaps the villian kaguya needed the zetsu army for.
As for plotlines, that is the easiest thing kishi could do. He could explore the plot line of naruto finding and learning about the uzumaki clan. Him finding survivors. Or them exploreing far off lands. As the ninja lands may just be 1 continent on the planet.
Then there is of course spinoff series kishi could easily do.
This is just my opinion though. There is no guarantee that kishi would make a new series as good as this series.
