Kikai Sentai Zenkaiger (2021)

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.

The most well known Super Sentai thanks to it's first american adaptation Mighty Morphin Power Rangers and the original first Super Sentai Series Himitsu Sentai Goranger.

It's the very first Super Sentai series what made many of it's successors possible. If you want to watch the original first season of the Super Sentai series. Kikai Sentai Zenkaiger is the 45th Season.

It's the 44th sequel of the series from Super Sentai right after Mashin Sentai Kiramager which is the 43th sequel. If you would like to watch the super sentai series chronologically.

Both are tokusatsu shows written by a well-established female writer about a late-teen/young adult man who is surrounded by characters voiced by famous seiyuu. The lead character lives his daily life with a surrogate parental figure who runs a shop, after losing his parents at a young age.

The secondary, seiyuu-voiced characters also just happen to include an old-fashioned guy, an animal lover who only likes cute things, and a more nerdy/motherly character in both shows. However, while they are both color-coded, the personalities do not align to their similarly-colored counterpart.

And while Kamen Rider Den-O has the characters traveling through time to stop the bad guys, Kikai Sentai Zenkaiger has the parallel worlds come to them instead.

Both are seasons of Super Sentai that have a heavy focus on being the anniversary of the franchise. Both seasons have a focus on Super Sentai being part of the bigger world/multiverse. Rangers can use the power of the previous seasons but having the gimmick corresponding to that season.

Both are seasons of a Toei tokusatsu franchise: Zenkaiger for Super Sentai and Zi-O for Kamen Rider. They have a heavy focus on being the anniversary of the franchise. Both seasons have a focus on the franchise being part of the bigger world/multiverse. They also have a gimmick that has the power of those seasons stored in them and they also transform into forms that are based on previous seasons' heros.

Both are seasons of a Toei tokusatsu franchise: Zenkaiger for Super Sentai and Decade for Kamen Rider. They have a heavy focus on being the anniversary of the franchise. Both seasons have a focus on the franchise being part of the bigger multiverse. These multiverse worlds correspond with a previous season of their franchise. They also have a gimmick that has the power of those seasons stored in them.

Actually, they are the 2nd Super Hero Toei in the Reiwa era, at the same time the timeslot is also the same and at the same time has crossover on the same day and movie.