In one of the rarest of cases, the sentai that will replace Zenkaiger, Donebrothers, will be a sequel to it, having actors/characters and world elements care over.
DonMomotaro already appeared in an episode of Zenkaiger and Zenkaiser will be a supporting character in Donbrothers with a new form to fight in. Also, the combined mecha that DonMomotaro and the Zenkaigers had will reappear in Donbrothers.
So, while it might not be absolutely necessary, it might be a good idea to watch Zenkaiger before starting Donbrothers.
DonMomotaro already appeared in an episode of Zenkaiger and Zenkaiser will be a supporting character in Donbrothers with a new form to fight in. Also, the combined mecha that DonMomotaro and the Zenkaigers had will reappear in Donbrothers.
So, while it might not be absolutely necessary, it might be a good idea to watch Zenkaiger before starting Donbrothers.
It's the 45th sequel of the series from Super Sentai right after Kikai Sentai Zenkaiger which is the 44th sequel. If you would like to watch the super sentai series chronologically.