In a now deleted Facebook post, the long serving Power Rangers casting director Iris Hampton seemingly confirmed that casting for Season 26 would begin in March:
The post is somewhat surprising due to the fact Power Rangers has yet to be officially renewed for another season by Nickelodeon. The deal between Nickelodeon and Saban ends with Season 25 aka Super Ninja Steel which has already wrapped production and will conclude later this year. Usually by this time, we would already have had a new season announcement and/or the trademark for the upcoming season (which would reveal its title and theme) would have leaked. Hampton's post is the first concrete sign of a new season being in active development.
Currently, there is no indication of whether the new season will air on Nickelodeon or another network, or what Sentai is being adapted (though Kyuranger is VERY likely to be adapted next). Hopefully we will get an official announcement for a new season soon!